Mount the Gardena MultiControl Duo on your wall or post rather than attaching it directly to the tap.

This model is inspired by and borrows a part from Frederik, the upper collar that rigidly locks the computer in place.

The upper collar is essential because the lower mount ‘basket’ has a little more clearance for removal and/or tightening the sockets from the outlets.

For the lower mount, Gardena has not made the two outlets symmetrical. The left outlet is a little closer to the edge than the right. The design accommodates this, but when you put it into your slicer, don't panic that it isn't symmetrical.

Attach the lower mount to your surface first, as the screws are hidden behind the Gardena unit once installed. Slide the upper collar onto the unit, ensuring the tap adapter is correctly aligned with the back face of the unit, then put it into the lower mount and screw the upper mount to your surface.

The lower mount has internal chamfers to allow water to flow away from the corners, but it may still collect some water. Treating the internal level surface with some epoxy resin will better ensure water won't collect in the ridges, and/or the Fusion360 file is available for you to add a draft, as below:

There is an STL with a 3-degree draft built in, but I have not tested if this helps or not.

Printing:
I printed the lower mount with PETG, 42% adaptive cubic infill, 4 perimeters, 0.3mm layer height using a 0.6mm nozzle. My print time was around 1h20m (but your time will vary).

Installing warnings:
Although the mount holes between top and bottom mount are the same spacing, they don't entirely line up vertically, so you'll need a mounting surface about 10mm wider than the hole spacing. Make sure you mark top and bottom mount holes before drilling/screwing.

You'll need a driver bit long enough to clear the model depth for the outer screw holes, as the cutout only really helps with the middle screw hole. Or just use an old-school screwdriver after drilling a pilot hole.