Most of my antennas for VHF and above, use T6 1/8" thick, 1/2" round aluminum tubing for elements and square tubing for the boom.
For 6m and 2m I recommend T6 1/8" thick, 1" square tubing for the boom. This stock is available from Ridalco on Michael Street near Belfast Rd (Ottawa).
Here is a picture of how I attach the elements to the boom.
The elements are fastened to the boom by attaching to 1/8 inch thick, 3 inch square aluminum mounting plates, using home made U-bolts.
The u-bolts are made from 6-24 threaded rod which is bent around a piece of 1/2" tubing. To accomplish this, make yourself a jig of 2"x 6" spruce. Drill the wood to snuggly accept a short piece of 1/2" round tubing. About 1/2" from the tubing, drive in a sturdy 3" nail. Now to make your U-bolts, wedge the threaded rod between the nail and the tube with long part of threaded rod extending past the 1/2" tube. Carefully wrap the rod around the tube. When you get it pretty much to a U shape, thread the rod, with the U at one end, down a super-champ tool (for cutting screws), cut the rod at a point where the legs of the U are long enough to fit through a plate while holding the 1/2" tube. Fabricate enough of these u-bolts for the entire antenna (10 U-bolts). This process takes a little experimentation, especially cutting the rod so that you can put a nut on it. Work carefully, the price is right and the U-bolts are excellent. The thread rod really grips the elements and holds them well.
All plates should be exactly the same.
For all the elements, fasten the plate to the element with 2 U-bolts. When mounting the plate and element to the boom, only fasten with one screw to the boom. Then, using a square (I actually build mine on the patio and use the patio stones to get everything square), make sure everything is square, before you mark, punch and drill that last mounting hole in the boom. Mark through the already drilled hole in the mounting plate. Now when you fasten the last screw, you know everything will be square.
NOTE: Because this antenna uses screws and not pop rivets, it is easily collapsible for transport.