MK IV tubular spaceframe drawing

I have now constructed an alternative variant of the side boxes.
Aluminum plates riveted to the tanks.
Weight saving approx. 16
These two tanks are bigger than we thought.
To make them removable I changed the side boxes again
Thanks to the McLaren group, Udo and Andrew for the great input
Hi ,
I have now completely redesigned my frame. Additionally I have done extensive FEM studies. I have always used 42crmo4 with 800-1000Nm tensile strength as material.
The tubes are 40x40x2 and 25x25x2 mm
Unfortunately I have to realize that there are only round tubes made of this material :(
Now I have only two possibilities to redesign everything with round tubes or to use another material.

Which material could you recommend me?