Extension Shelf bracket

Looking to do this myself, for a significantly larger piece - good resources, thanks!

a search for drop leaf table supports looks like what you want drop leaf table supports

Lee Valley is one of my go-to places for hardware and oddball tools. They offer this folding bracket which looks pretty good (and very sturdy) to me, at least based on the description. The price is, umm, commensurate with their apparent quality. They also have these and these rather more affordable variants that look like they may also work out fine.

If you're a little handy, you could also construct a wooden triangular support that is hinged along a vertical axis behind/underneath the shelf, much like many drop leaf tables use to support the drop leaf. Typically you'd have a little wedge shaped landing pad piece on the bottom of the shelf to make sure it's solidly propped open and won't flop closed at an inopportune time. I'd put a couple of felt bumpers in at key places, too, so it didn't rattle when folded on the road (at least for a motorhome--it's probably less of a concern in a trailer where nobody will be there to hear it rattle).

You might look here. Or try woodworker supply