You don't want to screw into the sidewall unless you know for sure where your cooling lines are. But, I think you are correct that a thin sheet of plastic and foam insulation is an inadequate anchor point in of itself. I use a dual reg setup in my dual tap so I don't have the mounting issues that you do, but my thought if I were in your situation would be to make a small wood frame to attach the manifold to. Possibly superglueing it to the sidewall if necessary. But, I'm sure others with more experience on this will chime in anytime now.