For the shapes of the climbing modules, I took inspiration from the simple circle-triangle-square shapes that were repeated throughout the 2021 Netflix hit, The Squid Game.  (I am still not sure what I think about this show.  It's brutal and disturbing and it doesn't let up.  It was exhausting to watch.  But it certainly was a hit.)

The acrylic half-spheres will stand-in for the circle.  The remaining climbing modules will consist of a pair of equilateral triangles and a pair of squares.  The faces of the modules have the same width as the raised edge of the trays so that visually, the tray edge will complete the missing edge of the square or triangle.

Each module (I didn't take any photos of their construction) will either be mounted on top of a tray:

...or beneath a tray:

I experimented with various layouts on basement floor to see what would be practical given nominal cat dimensions and the maximum range of mounting holes of the uprights.  I've pretty much determined that I'll only be able to fit 5 trays in the vertical space I have.  I want to be able to shift the modules around if/when I want, and laying them out in different configurations helped me work out a way to do that.  For your viewing monotony, here are some layouts I captured.  All but the first is doable within the usable height.

I noticed my tray construction blunder too late - I have the 'B' side of the plywood showing on the underside of the trays.  I should have put that on the inside since it will be covered by a carpet insert.   No big deal, since it is just cat furniture.  Besides, I may need to change some dimensions anyway after the cats put some miles on it, so there's always an opportunity for a rebuild.

All these components will get a coat of sanding sealer followed by some water-borne poly.