Modified an old Ansco tank with a groove to hold the movable flange at the right spot.
I do the same thing. If you use plastic reels, take an adjustable reel and set it at the point where the sheets will slide into the reel easily. Glue the reel at that point. You will have to work the sheets in to the reel since the ratcheting action won't work when the reel is glued. It's not a problem.
I can get six sheets in to the reel just by working them in with my fingers. Pop it in the tank, and process like roll film.
I have done it in 5x7 and 4x5 trays, and also borrowed my friend's Combi Plan tank, which you can get from Freestyle. I liked the tank, though it does use a lot of solution and take a while to pour. The film turned out fine, though.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."