wheel cups

Preformed recesses in the top of a road case, so that another case may be stacked on top, with its casters fitting into the recesses.


Penn Elcom : Hardware - Miscellaneous Castors & Accessories

Alternatively, if a case does not have built-in "caster dishes," donuts can be fabricated from two pieces of 6"x6" x 3/4" plywood stacked, with the upper piece having a 4" diameter hole cut-out.