It was very hard to find holders for 18650 sized cells, so instead, I bought some neodynium magnets, and covered them with copper tape. Works perfect as a battery holder! Lets me make various "pack" configurations. Here's a 7S3P pack. The magnets are attracted to the alligator clips and the cell bodies. I think using magnets with tape must have more resistance than a spring battery holder, but I don't think it's gonna affect the test results that much, since I'm not doing high discharge tests. I can do single cell, max 8 amp discharge tests, don't have the necessary equipment to do more.