Join Date: Jan 2019
Location: Central North Carolina
The most stable sheet wood product that I know of is "Tempered Hardboard". It is also called "Tempered Masonite". It comes in 4 X 8' sheets and is moisture resistant and very stable, but it's only about 1/8" thick. It's also quite reasonably priced. Your frame idea, either #1 or #2 would be the best approach for strength, but consider half lapping the corners instead of the butt joint. For a half lap joint you overlap the two pieces, but remove 1/2 of the thickness of each piece at the point where they overlap, so the result is a joint that is the same thickness as just one of the boards. This makes for a very strong joint if glued together, but does require some accurate woodworking. Another way would be to use either of your #1 or #2 methods and use dowels, biscuits or pocket hole joints plus glue to get a good strong joint. The panel could then get glued, nailed, or screwed to your frame. I have no idea what your planned use is for this, but some triangle pieces of 1/4" plywood attached to the back side of each corner with glue and nails or screws would further strengthen the joint, if their use wouldn't be visually objectionable.
Pleas look at this material in a local lumber source and decide if it will do the job for you. If not, please tell us more about your intended final purpose of this.
Last edited by CharleyL; 04-26-2019 at 05:43 PM.