In the uTube video for Geberit they show a slotted Channel that the top fixings hang off and the feet get clamped into. Is this slotted channel 41mm x 21mm unistrut?
Just use timber battens screwed to the wall and then bolt the geberit fixings to that.
I use 3x2 and cut the supplied geberit hex screws to length, so they don't penetrate the other side of the 3x2 and push it off the wall/floor. You have to predrill first though as you've lost the tip of the screw.
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