This is a simple 50x50x50 external tube connector which bolts can be used to make a frame.

Mounting holes on the original design are made for M6 bolts with a big head, however a .step file has been used so recreate as you'd like! However, to make a sturdy enough frame you do require strong bolts. 

These aren't as strong as internal connectors for tube, which I will design eventually, However they do support a decent amount of weight with a high infill. Preferably tapping threads into the metal for your tube is preferred however buying bolts larger than 60mm and putting a nut on the other side works also.

These are loose enough to easily slide onto 50mm tube if you require a better press fit please modify the design however a word of caution, relying on a press-fit design for these puts a lot of stress on the plastic and with my experience they decided to crack a lot when using PLA.

Enjoy!