If you try for rolls, you could use mailer tubes as your cores. I've got long rolls of paper that come on 3" cores, so I can vouch for that. You'd then just have to set up troughs for trays and do continuous processing. Or you could trough the roll in developer, then cut, then fix.

If you go back to holders, the book 'Primitive Photography' has detailed plans for 10x12 plate holders & you could adapt. It seems to me that you could make holders out of black foamcore board and black cardboard for darkslides. A staple gun & some tape ought to hold it all together. You could probably knock out half a dozen in an afternoon.