Uneven mosaic with glass table top cover...advice?

I will be making a round tabletop for the outdoors from old plates and such. Since this will be an uneven surface, I want to lay a glass top over the finished product, and use it for outdoor dining.
I'm thinking that the weight of the glass will be enough to keep it in place, shouldn't shift scratching the glass underneath. The idea of silicone rubber bumpers on the underside of the glass seems to be a logistical nightmare...given that this will be a somewhat uneven surface.
Or do I attempt to do the mosaic in reverse? Adhering to paper, wrong side facing me, then grouting my surface and laying it over? Might aid in a more level top?
Anyone with experience with this to help before I begin?