I've used both flat and button top in EPM's. Just depends on how far the ring is above the battery top. Personally I'd stay away from amazon batteries and go with AW / IMR bats. check out Tulsa Oklahoma Electronic Cigarettes Supplies. I just don't trust any name with "fire" in the title. Also check Home | ePipeMods Forum- The Forum Home of ePipeMods and the Greater ePipe Community for info on EPM pipes.
I use Efest flats and have never had a single issue with them either as batteries or in their compatibility with EPM's