This is a series of joining pieces for 1515 extrusion profiles. I've included some variants of T-shaped and L-shaped joints, as well as joints with screwdriver channels for blind connections.

I found that the outer wall is the most suitable for correcting joints. So far, I haven't had any problems using these joints to lift my Voron 0. I think the direction of the layer lines in the T-shaped joint makes them strong enough for this application. I wouldn't use them as the main source of strength for other mechanical applications or situations requiring high shear strength.

Despite that, I still think it's easier and faster than buying aluminum corner joints or special nuts. Using nuts that don't fall off can make your life much easier.