Using something like wooden curtain rod (1"-1 1/4" dowel) is easy. If it's a tiny bit
long so you push the cab walls out when you 'slide' it square across the dimension
you are spanning, it loads the sides nicely. You put it in place into two puddles of
glue, and wipe up once it's in place.

This isn't nearly as good as a full rectangular panel with big holes that braces
in two dimensions, but it's an easy retrofit and it works. I have done it several
times through the woofer cutout for front-to-back and side-to-side bracing.

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