There are 5c – er40 collet adapters available from ebay, but they originalte and are posted from the far east. Having the same problem as you I purchased one. It works, but it does stick out somewhat from the face of the spindexer.
The only caveat it that tightening the collet is a little akward due to the nature of the construction … you might consider a low-friction ball bearing collet nut like this one as a usefule improvement: