I'm installing sliding glass doors on a shower stall (aluminum frame with frameless glass). The aluminum side jamb vertical fit (on the shower head side) widens about 1/2 inch from bottom to top. What kind of shim material should I use behind the jamb (including the top screw of that jamb) to help level and reduce this gap.

I do plan to adjust the glass door runners to also help close this gap, but I am interested in the best material to use for a shim behind the jamb. I will also use a bathroom caulk to seal the vertical gap between the jamb and the tile wall.

Currently I'm planning on using the following as a support shim;
a 1/4" by 1-1/8" by 18" long piece of acrylic plastic with a hole drilled through it for the top jamb screw behind the side jamb and will caulk the sides and top edge of the jamb.

This will at least give a solid surface for the sliding door (little to no hollow area behind the jamb).