I think you will be fine on the R315 in a tallboy SGC slab, they are a little thinner than an exhibit but I believe still thick enough to not slip. I only have one but it looks great and fits just right. Just for reference I have had over 1000 exhibits slabbed at SGC and have yet to have one slip outside the gasket. Also on a card at that value I am pretty sure they will cut it a custom gasket, if your worried call Earl at SGC and im sure he will help.
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