I built an 8" HWK for some "cast off" drivers I had laying around and I used MDF, followed the instructions to the letter, except I flared the ports to start with and it sounded really good as a bedroom sub, very low power use.
Recently, I gave it to a friend who has a lead foot as far as the power goes and there were some resonance issues. I helped him sort it all out, by using Dynamat inside the top and bottom covers, along with incorporating the "BOB JOB" inside the iso chamber.
There was still some panel flex (MDF and partical board are known for ballooning under pressure) and his WAF was not a positive number, so he skinned it with a 1/4" oak face plywood and some trim pieces, thinking it might help the flexion, slightly, but it would balance the WAF equation. Again I helped out and instead of lightly tacking the skin, which he wanted to do quickly to appease her, I used a half a bottle of glue and made it ONE PIECE ... 3/4" MDF + 1/4" plywood. I knew the 1/4" would add some rigidity, but I did not understand how drastically the thin skin would help to stop the flex.
This baby HWK is now solid and sounds very defined and nonresonant. I doubt if the 1/4" material would have the same ability with the larger cab, but you might try some 1/2" glued on well, if panel flex is a problem. It makes it look really nice, too.