That aluminum plate fell off while I was driving under influence of something, or at least not paying attention, leaving the "free radical" to pivot around the bolts fastened into the rear dropouts. This gave a weird kind of flexibility over small humps in the road, but it sapped a lot of the already very limited supply of energy available for propelling this contraption in the desired direction.
I needed to fix this, and I needed to fix this fast. I certainly didn't have time to order parts from the US.
I considered using a piece of plywood. But while scratching my head and looking for odds and ends, I found some of my old school Stanley knives (one of them below, against a backdrop of heavily pollinated plywood). Note the duct tape, placed to avoid accidental wrist slashing.
The handle (and container of extra blades) on this knife is made of aluminum, so it weighs nothing, and is shaped in a way that makes it stronger than if it was flat. Best of all, it already had a threaded hole where I needed it to be. (That part about the weight is actually neither here nor there. This is a single speed load bike and some extra ounces, seriously, will not make a difference.) So I bolted it into place, extremely satisfied with finding an appropriate nut and washer. I do not have my shit together but at least my workshop is halfway organized.
I then sawed off the part that would have interfered with the pedals (below).
All in all, it worked OK. But i should add some pieces of bicycle tube to it, after a couple of miles it does slide around a bit, there being no real friction between the plate and the frame.