Ole Tjugen has a trick with jam to stick the film down to the plate. I have used roll-on post-it glue. There are film sheaths that get inserted into plate holders to replace the glass and hold the film in the correct position if sticking things down seems too... basic. The best solution would be to buy, borrow or trade some film holders. You will probably like 4x5 and want them anyway.

Good luck with the Seneca. It's a nice camera. You don't say what the lens is, but if it's the same vintage as the camera you may need to spruce up the shutter before getting too serious.