I've made a separate stand, out of wood. A sort of box on which I can screw my lever and handbrake. I'm also thinking of adding pipe clamps to it so that it's integral with the seat. This solution has the advantage of not bending under the stress of the handbrake and the sequential lever.

Personally, I have not yet attached a gearstick to my Playseat Challenge, but there are several tutorials on YouTube that show how to do it (https://www.youtube.com/watch).

Basically the seat has no support, but you can find some on different sites (not on digitec unfortunately). Here is an example: https://www.boulanger.pro/volant-ped...

It's just a metal bar that you screw to the seat and to which you screw the shifter.