Wheels are round because they work best which is why everyone copies them, no shame in copying if it's what works best.!! Adding at a later date never works out well or like you expect and is certainly not fun when it affects sensitive surfaces which are important to machine accuracy.
Hi Dean, I'm on with a new design - I understand the points you are making are valid.

Also, why 90x90.? That is an odd size for steel which I think you will probably find more expensive than 100x100 as it's less common.!
I agree there are certain places that work that way but I have found plenty that charge by weight and 90mm is cheaper than 100mm.

My own support frame is made of 50x50x3 which seems to be fine, so in principle, your round tube should be OK.
Hi Neale, That's good to know, and yes you are right I do need some triangulation in there.

Cheers Mike.