125mm extra (~5") isn't going to flex noticeably more than the existing size. If desired, I'd bolt a piece of 1/8" aluminum to the back of the extrusion for added strength.
Resizing the belts? Just get longer belts, assuming you can buy a belt the length you need from suppliers.
40w Full Spectrum Engineering 5th Gen Hobby 20"x12" w/ Rotary Engraver
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