Speaking only from my own experience, I've done the following: On the first one (in begginer mosaic days), I used quarter inch floor tiles I cut myself on the wet saw and used Mapei ready mix grout/adhesive to adhere them to the shoe, and then painted the grout w/glitter glue. For some reason, it didn't flex and crack the grout. I'm assuming it's because the tiles were so thick. On the second one, I filled the shoe w/Quickcrete, mosaiced it w/red glass mirror tiles, using GE II, covered the concrete w/green velvet. On the third one I used a couple layers of dry wall tape, then gave it a wash of plaster of paris, and another layer of dry wall tape, used GE II and it held up w/no cracking. I tried to mosaic the inside but it's a pain. IF I mosaic another shoe, I like the concrete inside method best. On the smell, try Frebreze. Didn't have that problem.

hmmmm I was wondering how you did that last one, Slow. Dry wall tape and plaster duely noted! As you know I filled mine with concrete after reading of your 2nd experience and that went fine. I had pondered setting the spring exchange as a shoe but worried about the weight and problems for everyone to stiffen their shoes.

Febreze is like a miracle isn't it!

Rigid Wrap is good for this application, too. It's plaster impregnated tape - like you'd make a cast for a broken arm with. I got my rolls really cheap at my local surplus store.

Your main concern is flexing in the leather (or vinyl). If made for exterior use (and I end up putting all sorts of things in my garden), the flexing will come from heating and cooling. Grout doesn't last long in that environment.

Best of luck.