Right I've made a decision!

I called around a load of my usual aluminium guys, Smiths, Aalco, Blackburns, plus a couple of local metal shops. No one has any 6x3x3/8 box on the shelf (even Aalco who have a warehouse near me about 10 acres in size) and although they could get it I would need to buy a full length. To be fair I didn't try Simmal.

I've built two previous machines using 6x2x1/4 box and they worked fine albeit I had to shim the rails, only very slightly as I didn't have a mill back then and I had to bolt blocks into the ends to bolt into (not required on the 3/8 box). Anyway I'm just going to go with the original plan from the first post in this thread, will order some 160x80 Item heavy clone from KJN tomorrow.

I will first try to just mount rails directly to the central flat on the profile, if this turns out too wobbly I will plate over the slots and fix rails to plate.
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