Most of my antennas use square booms for VHF and above. These are T6, 1/8" thick, 3/4" or 1" square aluminum tubing.
For 6m and 2m I recommend 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). I find Ridalco's stock very good but I suggest that you inspect each piece of aluminum to make sure that it is straight. Elements are not too big a problem but a slightly bent boom is annoying.
Here is a picture of how I attach the boom to the mast.
I prefer to use a 1/8" thick, 4" square, aluminum mounting plate and 2 ea 2" stainless steel U-Bolts. You may substitute chrome alloy u-bolts or muffler clamps. NOTE: Do not drill the plate until you have the U-bolts. Be sure to drill the U-bolt holes at least one drill size (maybe 2) oversize. If you do not do this, it can be difficult to slip the U-bolts thru the plate when standing up there on the tower. NOTE: The holes for the boom to plate fastening should NOT be oversize.
Position the U-bolt holes on the plate so that they are about 3/4" from the edge and 2.5" apart. Centre the U-bolts laterally on the plate. Mark the plate with a marker or pencil. When you are satisfied with the layout, use a punch or awl to dimple the metal in preparation for drilling the holes. Drill the 4 holes for the U-bolts and drill only 1 of the 2 holes for the boom (at this time).
When preparing the boom for mounting to the plate, drill both holes in the boom, but only drill one hole in the plate. Lightly tighten the single bolt holding the boom to the plate. Then, using a level, square, or other technique, make sure that the boom is 90 degrees perpendicular to the mast. I actually build mine on a tripod and mast pipe which is perfectly vertical. Make sure everything is nice and square, before you mark, punch and drill the last mounting hole in the plate. Mark through the already drilled hole in the boom using a sharp nail or awl. Now when you fasten the last screw, you know everything will be close to square.