I know that this is over a year old but I picked up some 40mm x 40mm x 1.6mm steel tubing today and it was a beast... Maybe if you went off road you would need thicker if you were heavier but with the rear suspension as long as you didn't do anything to weaken the integrity of the box frame like...
Just ordered these as the manual says 3/8th - Closest would be m10 me thinks - Could not find a decent price on Ebay for some strange reason. I want to build more than one bent and have a couple tie rod spares
https://www.aliexpress.com/item/33015839236.html