ᐅ Basement Toilet: Floor Construction (Temporary)

I want to install a toilet (+ sink) in the basement. I have a corner where the distance to a 100mm (4 inch) drain pipe is short. Running a water line to the spot is possible. This won’t be a regular bathroom toilet, just a "construction toilet" during the house renovation and later for occasional use in the basement (for example, during gardening). Therefore, I don’t want to tile the area, and pipes can remain visible. That’s not an issue.

My idea is to buy an inexpensive wall-hung frame system and a toilet bowl from the hardware store.

The drain connection can be done; I won’t need a SANIBROY-type system. However, the toilet will have to be installed about 10-20cm (4-8 inches) higher on a kind of platform to ensure proper drainage. This is exactly the problem. The top layer of the basement floor is a screed, with brick underneath (the rest of the basement is brick). The basement feels dry overall, but since it’s an old building, there is some inherent moisture. There’s no mold.

So, how could I build a platform? I can’t just lay wooden beams on the floor with OSB boards on top because the beams would get damp and rot over time. Since it’s a niche, it might be possible to mount supports on the walls and then build the step with OSB boards.

What ideas do you have?