Miata structural question

how structural is the area at the bottom/front of the rear fenders on a Miata, aka the area that tends to rust out quite often if not looked after? further to the point, how much would it sacrifice the structure of the car if I were to cut out the rust, box the area in (literally, shove square tubing into the hole, weld tubing to un-rusted metal, then make removable louvered panel to cover resulting box-shaped hole), and then mount a duct for rear brake cooling, where the rust hole used to be? (It's currently big enough to fit part of my hand into :( ) the rust goes through at least two metal panels (counting the fender itself), and I feel like at this point, trying to rebuild all of that structure to be as close as OEM as I can get it would be a massive waste of time, especially if I can future-proof the car in the process (just in case I decide to go wheel-to-wheel racing with the car some day)

It's fairly structural as far as I can tell, because you can feel the difference between a rusty and a clean car. There's a lot of metal back there as well - as you've seen, it's a lot more than just a pretty skin.

I'm pretty sure you can buy patch panels that include the inner structure, so trying to rebuild that isn't as hard as you think. Your plan is an interesting one and may not result in significant weakness if you tie that square tube in well. The wall thickness of the tube is going to be quite a bit more than the original metal.

thanks, that was sort of what I was worried about, since at least two pieces of metal in that area have holes in them, as well as the rear 8" of the passenger side pinch rail are completely rotted out (it makes lovely noises when you have to put it on a lift for servicing...). I hope that I can find patch panels that include the inner structure, since rebuilding all that stuff from scratch wouldn't be fun at all