Home Depot and/or Lowes have one type of an end cap with a screw mounting U slot in the center of it's base. You can just use a screw to attach them to the top of your canopy. I have 4 of these awaiting a new canopy I'll be building. If you're worried about it comming loose and sliding out from under the screw, you could glue a small piece of wood on the backside so it can't slide backwards.
And, glad you liked the design. I've built a simpler 15g high hood with the same general design, and will probably keep building them this way
