I've only played with this mod for maybe an hour, so perhaps there is something I'm not seeing, but I don't see a reason for the graves to be solid. Personally, I'd like them not to be.

The default model looks as if a player should be able to walk over it, so it's a bit jarring to suddenly walk into a barrier. Not using a barrier would also remove the need to replace a block and allow graves to be buried in blocks/fluids. Still check for an opening to place the grave, but no longer will there be missing snow layers or underwater pockets of air.