Kent,
I love the half plate format. I have a 'new-to-me' Underwood Instanto camera. Absolutely delightful camera. I learned a lot in the year it took me to find the camera. I'm happy to share a couple of tips I think are worthwhile. First, the term "half-plate" can be confusing. Whole plate is always 6-1/2 x 8-1/2 (inches). Quarter-plate glass plate is 3-1/4 x 4-1/4, so you'd think that half plate would be 6-1/2 x 4-1/4, but that size is usually called 'double-quarter'. The size that is almost always considered 'real' half plate is 4.75 x 6.5 inches. Back in the day, it was a very popular size and there were a multitude of camera makers, each with an ever-so-slightly different size and holders to match. And, the holders were usually book style (open on hinges.) You'll want to make sure you get at least one with your camera. Having said that, I have been able to adapt three different sizes to match my camera by shaving a bit off the sides and carving new notches (once you examine a camera and book style holders you'll see exactly what I mean.)
It's very easy to use film with any plate holder that's bigger than the film format. Cut a piece of 4-ply mat board to the plate size and cover it with black paper (or paint it black). Use a dab of re-positionable adhesive (Post-it type) in each corner of the non-emulsion side of the film and press it to the mat board. The film will stay in place until you're ready to take it out of the camera. Only trick is to make sure you know where the film is when you focus through the ground glass.
Best of fun!
d