Epoxy will be a terrible mess to work with. The best way would be to make a little mold, then file the cured epoxy to shape.
Here's what I would do -
if I'm out in the hinterland without easy access to aluminum or brass strip, I would get a plastic cutting board from Walmart or someplace similar, then cut out and file a spacer.
if I can get aluminum or brass strip, I would buy two or three thicknesses, probably 1/8th inch, 1/16th inch ,and 1/32nd inch thickness. For this application I would start with a 1/8th inch thick spacer, then if that didn't work, laminate a 1/32nd inch piece (with epoxy) to the 1/8th inch piece, and so on until I'm reduced to filing a laminated stack to a thickness that works.
after that, order a piece of nylon, teflon, polyurethane, or about any other plastic stock from McMaster Carr, or a piece of laminated shim stock if they carry that material.
If I was bound and determined to use epoxy, I think I would laminate up a stack of old blue jeans and epoxy, then cut that to shape.