Straight sections of box steel

[Smiler]

Personally I would go the route of just using the staightest box section you can find with the heaviest wall you can get. Make your frame and get it as square and flat as you can with whatever tools you have access to, for me, square being the most important. Bolt the whole thing down to concrete securely shimming as level as poss. Fit your rails and then shim them flat, level and parallel take your time and you'll be amazed how good a result you can get. If you have access to oxy/acet then you could try heat shrinking any bows out of your box but it's just not worth it IMO, shim the shiny bits instead.

Jeff.

[JAZZCNC]

Completey Agree with Jeff.
It's all about prep and patience not rushing being the key. You can get very very accurate with shiming and scraping/grinding, Don't rush the welding and keep the runs short to minimise warping trying to balance the weld location not heating one spot too much.

I also think you'll be surprised just how NOT flat or straight some of the Alu profile can be, even the better stuff. . . . .Or should say more expensive stuff.!!
If you want truely truely flat the both would need to be machined surfaced or ground.