This is a quick remix of the shelf support peg by DevopsPalmer: https://makerworld.com/en/models/178949

I changed the support to have a flat top for shelves that go flush against cabinet walls. The peg is made for a 5mm hole, but the fit is pretty snug. I had to wiggle the pegs in, but they are pretty secure!

I used a black PLA+ plus filament, which seems strong enough for my use (kitchen storage).