It would be a simple matter to plug the bottom of a six shell caddy at the edges with some delrin or something similar. If you leave the middle open you would still be able to grasp and index a finger on the middle of the bottom shell.
Thanks, that's what I'm going to do. Just ordered two six rounders and will attempt to modify. And for the inquiries, yes, I can grasp five shot shells at a time without issue. Just didn't want the added gap at the top when using six rounders.