Start with four of these:
Drill 100 holes in THREE of them and install a 2" shower drain in one that will become the bottom tray.
Cut 6- lengths of 3" pvc pipe about 4" long. This may vary depending on how deep your window boxes are. What they are used for is that a pair of them are placed on end in one tray and the tray above sits on them. This puts all the weight in the bottom and not on each tray. Less tippy. Partially fill the bottom three with 1 cu. ft. lava rock and put Matala or similar pads (we have quilt batting at the moment in the top tray. Feed water at the top and let it trickle down and out the shower drain.
Think I have about 35 bucks into it.
The cover is made from the companion tray made for the window boxes to sit on. Think it was $.99.