I actually have a set of these, new. There was a guy on the Vintage Computer Forums who ran a group buy to get new ones produced in the original dimensions. They're used on the bottom of old IBM PCs also, as I recall.
The material is kind of similar to that used in cork automotive gaskets, so if one could purchase a sheet of gasket material (I think FelPro sells such a package), it might be possible to make them that way. Not sure if the thickness is correct though.