Well, I've been thinking about how to build this for a while now. I wanted something that was inexpensive, functional, and super easy to assemble/disassemble each year (even without help). Here's what I came up with in photos:
A couple of 2x8s with perpendicular 2x6s to give me something upon which to hang 2x4s. The 2x8s are just resting on extra wall blocks.
The 2x4s are just hung in place with brackets so it's really easy to put together or take apart without needing two people:
Finished frame and covered with a solar pool cover:
I will hold the plastic down using extra wall blocks. I just set it on there for now for fitment purposes.
BTW, I was able to make this using only a hand saw and a drill which was nice because I don't own a circular saw. The wood and hardware was about $100. The pool cover was a semi-fancy 16mil solar cover I got on Amazon for about $80 but I think regular heavy duty plastic would also work.
Thanks to everyone who has posted their pond covers. Previous posts gave me the incentive, ideas, and confidence to make my own.