Hi Mike,
I used some aluminium for my repstrap construction. The corner brackets are made out of hardwood, and that does now seem to work. Used some ordinary wood screws to fixate the horizontal rods.
I made the Carriage out of 9mm birch multiplex with bronze/brass bearings from an old scanner. The idler bracket and motor bracket are made out of rectangular alumium tubes, 2mm thickness, measuring 20mm x 40 mm. The constraint bracket and diagonal tie brackets are made out of aluminium square tube, 1 mm thickness. For all the alu parts I drilled and tapped M5 holes, according to or approximating the darwin design, or used 6mm holes and M5 caps and nuts. Also the bearing brackets are made of this same alu tube with an M5 to fix the diagonal studs.
A few tricky parts remain: the motor mounting for Y and Z, finishing the motor mounting for X. All of the Z and table related parts are made of 19mm triplex. If you need any hints, please let me know.
Joost