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.