Ikea LACK 20 cm squaretube hanger.

For mounting the two parts together: 

2x M4 16/20 mm

2x cnckitchen's M4 threaded inserts.

There is a clamp for the 2020 squaretube in the bottom which needs a M4 screw and nut.

No supports needed but it is beneficial to use more than 2 perimeter.

4 should be ok (depending on the load you want to mount on the tube …)