do exactly what you're looking for, and they are a product that already exists. I have a set that I bought a few years ago, used once or twice, and am no longer interested in. If you'd like them I'll sell them for $25 shipped (payable via PayPal). They are in great shape, I just don't have any use for them so they are sitting on the shelf.
Note that these are not
a replacement for proper platforms, but they are good enough to clip on and ride up to the store, around the block, etc.