I have been wanting to get into 4x10 photography for a while now and came up with the following solution that according to my field testing this morning- it works! Sorry for the crappy pics (courtesy of my wifes litte point and shoot but they are only here to give you all the general idea). The system I decided to build is a 8x10 holder sized piece that in turn centers and holds a 4x10 holder. The conversion piece is a sheet of hardboard that has been sized to mimic an 8x10 holder. It has a groove milled into it that catches the ridge on the 4x10 holder (done using a table saw). Another two pieces of hardboard are used to sandwich some adhesive velvet (hobby store- commonly used to line jewelry boxes) around a pair of blocks that are planed to provide a total thickness (when coupled with the smaller hardboard pieces_ to exactly match that of the 4x10 holder. I made these a bit large with the idea of clamping them to the backing piece with a 4x10 holder in place to provide and exact fit when the whole assembly was screwed together. I then trimmed the excess from the edges to give this holder. Total cost- about $5-10 and a couple hours of time. The velvet was masked with tape and the innards were spraypainted with matte black and the device was tested first in my darkroom in place on the camera with flashlight looking for leaks- none found. The final test was this morning- with no leaks even using a long exposure. I'm sure something like this has been done before but it was fun and best of all, I can shoot interchangably between 8x10 and 4x10 with only a small amount of added weight (14 oz) added to the pack- hope this gives someone else a good idea- it really wasnt too tough to make.