This is a handful of joints to use with 15mm x 15mm aluminum extrusions. I included a few variations of T- and L-joints with screwdriver access for blind joints as well.

I've found that the outer walls work the best for squaring the joints. So far I've been able to pick up my Voron 0 using these joints without issue. I think the direction of layer lines for the T-joints makes them strong enough for this type of application. 

I wouldn't use them as the main strength for other mechanical applications or situations where they need to provide a lot of shear strength. Nonetheless, I still find this to be easier and faster than buying aluminum corner joints or specialized nuts. Combine this with no-drop nuts to make your life easier.