I was browsing Amazon and came across this 20mm linear rail kit for $200.  It is a great deal for such large rails, I could not come close to the price looking on AliExpress.  It makes me wonder what the catch is!  Anyway my linear motion cost around $140, so this is only $60 more and would be the basis of a larger and stiffer machine, assuming it works as described.  

Of course it is not designed for 3D printed parts, you would need to do some work to come up with a different frame system, probably plywood or aluminum sheets.  That cost would need to be factored in as well, and you probably want to up the stepper motor and controllers as well since were going bigger.  Chances are by the time your done your in for another $300, but you would have a larger and stronger setup.

It may make a good upgrade path to incrementally grow the MultiBot into a more serious machine.

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Here is another similar offering for almost the same price.