Forget about trying to match the T dimension. Unless you're willing to have holders custom-built to match your camera, you'll just have to take what you can get in that respect. For basic fit, I believe the Seneca should be OK with Eastman-type holders, which are the most common type of older WP holder. Anyway, check the holder's external dimensions and/or buy with a return privilege and you'll be fine. Also, don't forget that plate holders with film sheaths are a viable alternative, in case you happen to run across those first.
Good luck, and enjoy!