Get a tennis ball or similarly sized rubber ball and cut it in half. You can actually find a wall protector at hardware stores that works perfectly. That's what is shown below. It is used as a door stop to protect the wall from a doorknob going through the sheetrock. Anyway, take two hemispheres and put them on either side of the magnet vent hole before fastening down your access panel. It will act as a brace for the panel.

I always hated access panels because they are prone to vibration. Any panel large enough to fit a 10" or 12" speaker through is large enough to vibrate. I don't care how rigid the rest of the cabinet is, and it doesn't matter if the panel is very solid, it always seems to be a trouble spot because you can't put a fixed brace in place.

I put wedge shaped braces on each of the four sides of the horn flare, so it and the cabinet surrounding it are solid as a rock. But without these rubber braces, the access panel wouldn't have any other mechanism to brace it. Put these in there and squish them down when you tighten the panel and viola! - The panel is preloaded and well braced. It will make that back panel as solid as the rest of the cabinet. You'll really hear the difference in the low keys of the piano, so do this step for sure.