I tried the 2x5 trick and it works really well. Being tool challenged I bought the modified darkslide from Bender Photographic. It costs around $20.00.
I have a couple of old 5x7 film holders that are in bad shape so I might try my luck in in making a mask for 5x7. That will give two 2 1/2 x17cm (well almost) pano negs on one sheet.