tree slices for centre pieces - advice please

Good morning everyone, I'm after a bit of advice. My OH really wants to place our lanterns on top of a thick tree slice, a bit like this (our lanterns are different but you get the idea). We have spent months trying to source these locally (who knew they would be so difficult to get hold of!!). Anyway yesterday The FMIL came over with a sample slice she had managed to get from a sawmill. It is just what we want so she ordered 15 plus a huge one for the cake. Smiles all around. Until last night when my OH noticed the slice had started to crack from the centre! We knew this would happen over time but didn't expect a crack to appear after only a few hours! I know we should have put it in a cool place straight away but we didn't because we were busy. Anyway the FMIL is picking the order up on Tuesday. We are now worried that the slices will crack right through before our wedding at the end of December. Does anyone have any advice how to prevent this from happening other than keep them somewhere cold and dark like the garage?