Internal volume is the key to any box. You take your measurements and times them.. 1x3x7 then divide by 1728... or measure the outside and deduct 2.5 from your measurements. Again, using thicker MDF I don’t think would be an issue.
Just make sure where ever your calculations differ you add in the 1.25. And that will have some sturdiness to it. I would also keep the same amount of braces! Strong is one way to look at it, but with given enough force those 1.25 will vibrate. The point to braces is to help stop those vibrations that can cancel stuff out. Stored energy. That’s bad ummkey. Lol.
So keep the braces. Just be happy knowing you have the strongest sub box known. Lol.
