In designing this I knew I wanted it stiff and to maintain a level rolling area, so I made the infill at 50%, the top layer is 5mm with rectilinear surfaces top and bottom and a gyroid pattern for infill.  The top layer has to be thick for the roller to be most effective.  I'm very new to 3d design and used Tinkercad as I do for most everything I design. The stones fit pretty tight, so I gently used an emery board to take down a few thousandths from the ends of the plastic the stones are mounted in.  After a few test fits, it dropped in snuggly.  I made a hole in the one end so I can utilize the wing nut that the Lansky stone holder comes with to further lock the stones in my cradle.