I thought about mounting a board but what I ended up doing instead was took some door weather stripping mounted it on the inside on 1 door so when it closes the other door pushes against it creating a seal... along with that and some extra foam insulation it worked pretty well....

I wish I could do a padlock setup but I kind of wanted to go for more a stealthier look so im gonna try some basic keyhole locks with latches on the back for the top and bottom which shouldn't stick out to bad..

I agree with you on the cardboard backing it sucks... Right now I think it might work for me since at the most i might mount a couple 80mm computers fans on it but for my next grow(gotta get through this one first lol) Im def gonna look into a better intake/exhaust system and I dont think that back will hold up so ill be looking to replace that also..

Hope your grow goes well