Dont worry about the plate tourquing on you. One of the guns i do this on is about 35# and is in a very large 338 cartridge.
In my case the plate is mounted at the balance point of the gun and it supports the entire gun weight without help of any rear support.
The sides are grooved, and the whole thing slides into a return to battery rest on the bench. Entirly different use than you need, but the plate part would work for you.
The spacers should be at least 1" in diameter and 1 1/4" wouldnt hurt. Thats why i mentioned fender washers as they come in different sizes with different hole sizes.
1/4" bolts with allen heads will suffice to fasten the plate. No reason you couldnt experiment using wood also for the plate. Eventually you'll get it right.