For reference, in the video I use 2-12" and 4-24" or 30", plus a couple of 12" bar clamps to hold front and back while clamping with the parallel clamps. Parallel clamps are kinda spendy, but can be used a lot of ways. I guess it depends some on what projects might be in your future.
If you're looking to add to your clamp collection, I find the 12" fairly useful. If you were looking to add a pair to complement what you already have, 30" or so might give some future versatility. Longer than that just isn't necessary on a project this size.
FWIW, all the parallel clamps I have are Bessey, most bar clamps and pipe clamps Jorgensen. There are others out there, but most of these were purchased when the available clamps were fewer.
