At the last moment (walking into the hardware store), I decided to do the simplest build - option #4, the plywood and 2x4 contraption. It's also the cheapest.
So I grabbed a 4x8x3/4" sheet of plywood, and asked the folks at Home Depot to rip it in half lengthwise. Then I grabbed 7 2x4's. Four were left 8' in length to run along the sides on top and bottom of the frame. The other three were cut into 2' lengths - eight of those for legs, and then 4 of them for cross-braces at the ends(*1).
Two things I learned as I was getting stuff together at the store:
(* 1) Thinking through the necessary cuts in my head, I forgot that not everything needed to be 2' long; the cross-braces on top and bottom of the narrow ends should be 2' less 2x(thickness of a 2x4); I'll fix the m with a circular saw (Skilsaw) when assembling tonight.
(2) I thought I could get all the cut lumber into my car - a 1999 Toyota Camry sedan - because the two back seats actually fold forward, which would open up the passenger area and trunk to some storage. However, I've only folded down one of the two halves of the seat in many years, and I could only get that one unlocked and folded down. So, I couldn't get the 2'x8' pluywood sheets in there, and had to place them on "will call" at the store to pick up when I have a bigger vehicle. Oops :) I could get all the 2x4s in though..
Tonight, my 4 year old son and I will build the frame from 2x4 pieces, and then I'll pick up the plywood sheets tomorrow to screw onto the assembly as a lid.
Also, I'll stop by the local Harbor Freight and see if they have any wheel chocks that I could bolt to the front end for some additional stability. I have one or two other upgrades in mind, but we'll get the basics in place first.