These are some heavy duty corners I designed for the 25 mm version of the MPCNC
You can find the original design here:
Those are a bit stronger and should help to limit the wobble of the structure in case you have issues with it (it's rarely absolutely needed, the original design is good enough, but it might help if you're using the MPCNC as a printer and want to go for high speeds).
The corners feature some belt tensioners, you can adjust the belt tigtness just with one screw on each end.
I suggest you to print the corners using standard PLA, which is one of the most rigid material available. Don't go for materials who deform easily like ABS or PETG, otherwise printing these corners will be pointless.
It is using captive nuts all around, I recommend also to use some threaded inserts for the belt tensioner's screws
None of those parts need supports and for the infill obviously if you want rigidity then the higher the better.
They use the standard corner spacer originally designed by allted. I didn't include it here since it is not my design, but you can find it in the link above (name is "spacer corner")
I've had them on my machine for about two years and they work very well, it really improved my printer's stiffness.
For those who want to see how I build my machine, here it is: https://www.v1engineering.com/forum/topic/my-mpcnc-made-in-china/
Fairly easy to print, no big overhangs and not support needed. I recommend you to print hotter than usual, you want the best possible bond between layers in order to get the strongest possible part. Personnally I printed it in PLA at 215 Degree C.