Considering how scarce and expensive an 8X10 Sinar P back is going to be, I think you have a good idea in adapting another 8X10 back. I believe with the 8X10 P you run into that quirky Sinar thing of metering and non-metering backs. I don't know for sure, but I think maybe you need the correct version depending on which frame you have. I've read there are metering and non-metering bellows, which implies a difference in the frame. The only online Sinar catalog I have a link to shows only metering backs and bellows for 8X10, but I believe there was an earlier non-metering camera available. If any Sinar Purist catches you using a "wrong" back, say it is just temporary until you can find a genuine Sinar one.
A wooden back will save some weight, although with something as heavy as an 8X10 P, saving a pound or two may not be noticeable.
A wooden back will likely be easier to find than a metal one like a Cambo or Calumet, and less expensive. If you get a really ancient wooden back there is a chance it was designed for glass plates and the ground glass spacing may not be correct for modern film holders. This spacing can be adjusted though.
I did a quick look on eBay for stray 8X10 backs and they seem scarce at the moment. If you get impatient, you may have to buy an entire poor condition 8X10 camera just to get the back. Since new bellows for 8X10 cameras tend to be expensive, folks shy away from a camera with a bad bellows.
If you proceed with your idea, please post the results. I've considered doing this to get a workable 5X7 rear standard for my 4X5 Sinar Norma. I've seen 5X7 Norma rear ends missing the back, but passed them up hoping to find one complete with back. But I keep getting outbid, so I'm tempted to buy the next backless one I see and hang another back on it.
Len
