Summary: Drill bolt holes in two trapezoids, and mount them to the tray and masts.
Drill body holes for 1/4" bolts in the two gussets, as shown in the Hole Location drawing. In one gusset, drill one of the bolt holes distinctly out of position, so that when you dismantle and later reassemble your evapotron you can't mount the masts on the wrong sides. Note: these four bolts are inserted and removed repeatedly; this will be easier if you clean out the 1/4" holes in the gussets and the 2x4's, by pushing the drill bit in and out several times.
Position a gusset on the outer face of the tray, over the "deep end" shims, centered 15" from the tray's outer end. Align the gusset bottom edge with the 2x4 bottom edge (not the plywood floor's bottom edge), and clamp it in place. With the gusset holes as a guide, drill only one bolt hole through the 2x4. Insert a carriage bolt from the inside of the tray and tap it home with a hammer. Add a wing nut and hand tighten. (A fine detail for bolts that will be repeatedly inserted and removed: a carriage bolt has four square shoulders under its rounded head. These form a square indentation that's worth preserving. When re-inserting a bolt, match the indentation's orientation; better yet, always insert bolts with one shoulder facing directly upward.)
Repeat with the other three bolts; then repeat on the other side with the other gusset and mast.
Stand a mast on the tray rim, centered on a gusset; clamp loosely while you square the mast with the tray, then tighten the clamp. Drill a 1/4" hole through the mast, insert a bolt from inside, and add a washer and hex nut. Repeat with the second mast bolt.
Repeat with the other gusset and mast.