diff --git a/Lib/ufo2ft/__init__.py b/Lib/ufo2ft/__init__.py
index 72102292..5b77dd97 100644
--- a/Lib/ufo2ft/__init__.py
+++ b/Lib/ufo2ft/__init__.py
@@ -1,5 +1,4 @@
 import logging
-import os
 from collections import defaultdict
 from enum import IntEnum
 
@@ -7,7 +6,7 @@
 from fontTools.designspaceLib import DesignSpaceDocument
 from fontTools.designspaceLib.split import splitInterpolable, splitVariableFonts
 from fontTools.misc.loggingTools import Timer
-from fontTools.otlLib.optimize.gpos import GPOS_COMPACT_MODE_ENV_KEY
+from fontTools.otlLib.optimize.gpos import COMPRESSION_LEVEL as GPOS_COMPRESSION_LEVEL
 
 from ufo2ft.constants import SPARSE_OTF_MASTER_TABLES, SPARSE_TTF_MASTER_TABLES
 from ufo2ft.errors import InvalidDesignSpaceData
@@ -27,6 +26,7 @@
     _notdefGlyphFallback,
     colrClipBoxQuantization,
     ensure_all_sources_have_names,
+    getDefaultMasterFont,
     init_kwargs,
     location_to_string,
     prune_unknown_kwargs,
@@ -110,6 +110,7 @@ def call_postprocessor(otf, ufo, glyphSet, *, postProcessorClass, **kwargs):
     colrAutoClipBoxes=True,
     colrClipBoxQuantization=colrClipBoxQuantization,  # Callable[[Font], int]
     feaIncludeDir=None,
+    ftConfig=None,
 )
 
 compileOTF_args = {
@@ -841,6 +842,7 @@ def _compileNeededSources(
         if source.name in sourcesToCompile:
             sourcesByName[source.name] = source
 
+    ftConfig = kwargs.pop("ftConfig") or {}
     # Compile all needed sources in each interpolable subspace to make sure
     # they're all compatible; that also ensures that sub-vfs within the same
     # interpolable sub-space are compatible too.
@@ -850,25 +852,25 @@ def _compileNeededSources(
         if not subDoc.sources:
             continue
 
-        # FIXME: Hack until we get a fontTools config module. Disable GPOS
-        # compaction while building masters because the compaction will be undone
-        # anyway by varLib merge and then done again on the VF
-        gpos_compact_value = os.environ.pop(GPOS_COMPACT_MODE_ENV_KEY, None)
-        try:
-            ttfDesignSpace = compileInterpolatableFunc(
-                subDoc,
-                **{
-                    **kwargs,
-                    **dict(
-                        useProductionNames=False,  # will rename glyphs after varfont is built
-                        # No need to post-process intermediate fonts.
-                        postProcessorClass=None,
-                    ),
-                },
-            )
-        finally:
-            if gpos_compact_value is not None:
-                os.environ[GPOS_COMPACT_MODE_ENV_KEY] = gpos_compact_value
+        # Disable GPOS compaction while building masters because the compaction
+        # will be undone anyway by varLib merge and then done again on the VF
+        gpos_compact_value = ftConfig.pop(GPOS_COMPRESSION_LEVEL, None)
+        ttfDesignSpace = compileInterpolatableFunc(
+            subDoc,
+            ftConfig=ftConfig,
+            **{
+                **kwargs,
+                **dict(
+                    useProductionNames=False,  # will rename glyphs after varfont is built
+                    # No need to post-process intermediate fonts.
+                    postProcessorClass=None,
+                ),
+            },
+        )
+        if gpos_compact_value is not None:
+            baseTtf = getDefaultMasterFont(ttfDesignSpace)
+            baseTtf.cfg[GPOS_COMPRESSION_LEVEL] = gpos_compact_value
+            ftConfig[GPOS_COMPRESSION_LEVEL] = gpos_compact_value
 
         # Stick TTFs back into original big DS
         for ttfSource in ttfDesignSpace.sources:
diff --git a/Lib/ufo2ft/outlineCompiler.py b/Lib/ufo2ft/outlineCompiler.py
index 5b6d8511..1b7adf5a 100644
--- a/Lib/ufo2ft/outlineCompiler.py
+++ b/Lib/ufo2ft/outlineCompiler.py
@@ -108,6 +108,7 @@ def __init__(
         colrLayerReuse=True,
         colrAutoClipBoxes=True,
         colrClipBoxQuantization=colrClipBoxQuantization,
+        ftConfig=None,
     ):
         self.ufo = font
         # use the previously filtered glyphSet, if any
@@ -126,6 +127,7 @@ def __init__(
         self.colrLayerReuse = colrLayerReuse
         self.colrAutoClipBoxes = colrAutoClipBoxes
         self.colrClipBoxQuantization = colrClipBoxQuantization
+        self.ftConfig = ftConfig or {}
         # cached values defined later on
         self._glyphBoundingBoxes = None
         self._fontBoundingBox = None
@@ -136,7 +138,7 @@ def compile(self):
         """
         Compile the OpenType binary.
         """
-        self.otf = TTFont(sfntVersion=self.sfntVersion)
+        self.otf = TTFont(sfntVersion=self.sfntVersion, cfg=self.ftConfig)
 
         # only compile vertical metrics tables if vhea metrics are defined
         vertical_metrics = [
@@ -1104,6 +1106,7 @@ def __init__(
         colrLayerReuse=True,
         colrAutoClipBoxes=True,
         colrClipBoxQuantization=colrClipBoxQuantization,
+        ftConfig=None,
     ):
         if roundTolerance is not None:
             self.roundTolerance = float(roundTolerance)
@@ -1119,6 +1122,7 @@ def __init__(
             colrLayerReuse=colrLayerReuse,
             colrAutoClipBoxes=colrAutoClipBoxes,
             colrClipBoxQuantization=colrClipBoxQuantization,
+            ftConfig=ftConfig,
         )
         self.optimizeCFF = optimizeCFF
         self._defaultAndNominalWidths = None
diff --git a/Lib/ufo2ft/postProcessor.py b/Lib/ufo2ft/postProcessor.py
index 56f2292c..a7c25d18 100644
--- a/Lib/ufo2ft/postProcessor.py
+++ b/Lib/ufo2ft/postProcessor.py
@@ -404,4 +404,5 @@ def _reloadFont(font: TTFont) -> TTFont:
     stream = BytesIO()
     font.save(stream)
     stream.seek(0)
-    return TTFont(stream)
+    # keep the same Config (constructor will make a copy)
+    return TTFont(stream, cfg=font.cfg)