RM> is there an economical way of buying or
RM> making a 2 or 3 metre long straightedge?

RS> I think the best way to get that is just some heavy gauge aluminium
RS> channel, say 75x25mm. U shaped but with the 75mm the bottom of the U.

RM> Sounds like a good start, although I'm after something
RM> that's accurate to within 1 mm over 3 metres,

Should be fine for that on the 75mm dimension

RM> that I can use to make flat this sleeper benchtop I wanna build
RM> once the wood's dry. Perhaps if I build an aluminium truss over the
RM> top of the channel, with screw adjustments at each attachment point.

You shouldnt need to. Thats basically what the long levels are, tho they
are normally a closed section of alumunium with about those dimensions.

RM> Making slop-free joints could be fun, though.

Yeah, I've never been into doing that sort of thing with wood. I just
cut to the chase and do the structural stuff in steel, FAR better IMO.
@EOT:

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