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. For all others, I suggest that you use 3/4" 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 key is to use 1/8" aluminum mounting plates. For 2m and 222MHz, I used 2" square plates. For 70cm and above, I use 1" square plates.
Aluminum pop rivets make an excellent fastener as long as the elements are not too heavy. Aluminum pop rivets will make an excellent electrical connection between the other aluminum pieces, and you will not have to concern yourself with electrolysis. Always choose a sturdy aluminum rivet for fastening. Use the right depth rivet to extend through the mounting plate and into either the element or the boom, by at least 1/16" more than its thickness (1/8" for the larger rivets).
Select a suitable distance from the corner of the plate for the mounting holes. All plates should be exactly the same.
For all the elements, fasten the plate to the element with 2 rivets. When mounting to the boom, only fasten one of the corners to the boom. Then, using a square (I actually build mine on the basement floor and use the floor tile to getting everything square), make sure everything is nice and square, before you mark, punch and drill the last mounting hole in the boom. Mark through the already drilled hole in the mounting plate. Now when you fasten the last pop rivet, you know everything will be square.
NOTE: If you want this antenna to be collapsible, use long stainless screws to mount the plate to the boom.