Tried to find black high density closed cell rigid foam like the blue but in black. And find a black bottle cap. Neither the scrap piece of blue foam I tried or the red Coke bottle cap took black RIT dye very well.
Final proof of concept shaping the blue foam (the two discs eventually get glued together to get the width I want). These 4" tires on about 1-1/8" bottle caps are true to scale on the HZ Super Cub to the real 35" tires on 10" rims Alaskan Bushwheel makes... it's their largest product.