Solarbird 
having this assembled in a big block object makes it SO much easier to attach holy crow
definitely no need for the bungie cord here
may need to thin the lower pad a bit if anything
let’s see about the noise now
YEP still loud but again much quieter, like before.
okay so what've we learned
1: the swivel wheel works! and pretty well. It's DEFINITELY VERY NOISY, but not comedically so.
2: the bungee cord solution is both quieter and handles over any weight this thing should be carrying anyway, BUT!
3: if we're in a situation where we need to cargo something actually kinda heavy (like over 40kg/90lbs, which is more than the thing was built to carry anyway), then it's worth having the wheel. Maybe build a reinforcement plate to stick in that'll firm up the cargo bed a bit, since it's just plastic.
But in all other cases, using a bungee cord to transition some of the load to the far side of the cargo structure is a clear winner.
(I found basically this model, and it's rated for up to 40 _moving human child_ pounds, which is to say in more general terms 40 dynamic pounds. You can go a lot higher if the weight involved is static, which this of course would be. 80 static pounds is a guess, but it's not terrible either. Plus, I can obviously can run the safety limits tighter because it's not a child.)
So yeah basically I think that's a wrap for this project? I have a bike cargo carrier now, the best I can do for the time being. It can carry a pretty good load - more than we ever get just stopping by the grocery, and from that first trial we know it's more than enough to use for dedicated grocery runs.
With the either bungee solution _or_ the wheel solution, I'm not putting any visible (and therefore any meaningful) torsion on the frame, which means my hitch adapter works and is safe to use, so I can be pretty confident I'm not doing any harm to my bike with it.
Which was, of course, by far the most important question.
So yeah, I guess it's finished! I've put the third wheel into the bike accessories drawer, I've put the cargo cover I sewed onto the higher-accessibility accessories shelf (first order retrieval is my friend), and ...
yeah! I guess we're done!
(Well... other than maybe sewing a reflector onto the cargo cover. It's pretty damn visible with that safety orange cover, but... just to be sure. xD )
Took me long enough, but hey, I got there. :D