Aluminium 3030 extrusion profile corners made for extreme conditions ;-)
Internal tolerance is set to 0.15mm to avoid the need for hammer force to put it together. If you don't mind a few shorter pieces of spaghetti inside and your printer is able to print few centimetres long bridges, you won't need any supports to print them.

If you need any other shape of the corners, leave me a note and I'll upload them asap.

UPDATE:
T-shape added

Creality

Ender 3

No

No

0.24