I have swapped the plywood sheet for 7 planks of 1x4 I had in the basement. I cut these to 48" in length in order to utilize some of the old frame of the stroller as a side bar assembly. I'm not sure I like that, and I may just utilize it as a flat trailer and attach a few D-rings to the top to which I can attach bungee cords. Most of my use of this won't be for loose items like I'd put in a shopping cart. Instead, it's mostly for larger items that I can't otherwise carry, like folding chairs to go to the park, a bocce ball set, Weber Q grill, stuff like that. That sort of stuff can be easily bungee'd to the bed for secure travel.
I rode around with my 10 year old daughter on the trailer for about 2 miles last night. The trailer's springs are rated for 100 pounds total, and she's about 65, so it was a nice middle weight. I estimate the trailer itself to be about 25 pounds, so I was pulling 90 behind me. That'll build muscle fast. Something like this is on the opposite end of the spectrum to the notion of counting grams on bicycles.
I did an 8 mile ride this morning with the trailer empty. It pulls great. I do think I'm going to remove the side bar assembly completely, and shorten the bed a little bit. It's a full 48" right now. I may trim 6-8 inches off the back of it, to get it closer to the 40" range. It's not bulky to tow (you don't feel the length), but it's a little bulky to store, and 48", plus tow bar length, is a bit long/tall to be carrying around in the garage/basement.
IMG_20190622_100349960_HDR by
jnjadcock, on Flickr
IMG_20190622_100405586_HDR by
jnjadcock, on Flickr