I think you did great, the idea of using a plate is good, but you could just use 1/2" Fender washers, should give ya plenty of support
and you could just drill a small hole through each bolt just below the wing nuts and use a cotter pin, or one of the neat spring pins, just check out the hardware section of your favorite hardeware store.
Mary K:
I think Al Fink is on to something with his comments.
Fender washers are good and I have seen bolts with cotter pin holes in them, already made. You can probably find them at the hardware store. If not, I know Red Trailer has sold cotter pin bolts so Harbor Freight or someplace like that probably sells them too. If you put the wing nut on first and the cotter pin on after; then, if the wing nut comes loose it won't fall off. If you forget to check your spare, you'll get a rattling reminder instead of having it fall and drag on the road. It might also be a good idea to epoxy the drilled holes with a Q-tip, or put a dab of roof asphalt or something to prevent water-seeping rot. A little silicone sealer on the top side wouldn't hurt, either.
I'm with you on attaching the spare to the under-used bottom side. I haven't done it yet but I think that's where mine is going. Those little guys aren’t heavy. If you have the 12” size like mine, I weighed it and it was only about 10 pounds. A Thanksgiving Day turkey usually weighs more than that. It’s like you said: if you've got to change a tire, you're already going to get dirty and aggravated. Just keep a small plastic drop cloth in your tool kit in case you stop in a cold, wet mud puddle!
By the way, this is a good reminder for I need to do later, when I put mine on! Thanks!
(Among the greatest things about this forum is the collection of knowledge and ideas that come forth. I think we all build and survive better because of it.)