This page details a little how the circuit was made to fit onto a 27 x 52 mm prototype pcb and soldered to become a board ready to plug on the RPi's 20x2 GPIO pin set.
The pcb is sawed off to achieve the desired dimension, with 20x10 holes. Two sets of breakout pins will be soldered to the board: a 20x2 female block for the RPi GPIO, and an 8x1 male set for the two proximity sensors.
The female block faces "downwards" and the male block "upward", as in the following sketch:
The precise location of each pin, each connection and each blob of tin is detailed here below. One should be careful to think carefully the right order when soldering (e.g. first the ground row, then the echo resistors, then the LED sets, then the male breakout and finally the female breakout).
Soldered board:
Plugged on the RPi:
One could restructure the above layout to make the board still fit as an add-on to the RPi, but inward-facing, i.e. almost completely hidden into the Raspberry case (except for the sensor pins and the LEDs).
This exercise is left to the reader (because that was the original plan, but due to a major overlooking I ended up with the board orientation as shown and now I will happily stick to it).