Greetings from Norway.
My friend had similar obstructions behind his '78 horns, you know, radiators and stuff, so we acted on the Dope from Hope.
We built the corners "shoulder high", just as high as the bass bins. This was not for cosmetic, but for practical reasons; he has the tops twisted maybe 10-15 degrees outwards, just bolted down on the outside bracket, so that they focus a few feet further back. You can't do this if you build your corners all the way up.
This is one worthwile tweak! Bass used to be all over the place, now it's tidy and tunefull. Just getting the gaskets to make wall contact seemed to gain an octave.
Stein