Get a copy of Alan Greene's "Primitive Photography". It has a detailed section on making film holders that is very good. Another idea is Otto Dippold's manuals - he wrote them on both camera making and film holder constuction. Otto used to sell them on eBay- but I haven't seen them there in a while.

Another take on the xray film thing- use lith film and develop it in exhausted dektol. Freestle has a bunch of 11x14 lith films that won't break the bank and yield a very nice continuous tone negative- but exposure times will be long.