I used 3/4" plywood and then 1/4" hardi-backer board (cement board) mortared and screwed down to the plywood, just like you would do on a sub-floor in your home. Probably overkill, but I will never have to worry about grout lines or tiles cracking. Keep in mind that the tops on most coffin keezers are very heavy, especially when tiled. When you walk on the subfloor of your home, there is flex in the plywood, so for home tiling applications, mortaring down cement board and screwing every 6 to 8 inches in the face of the board gives the added stiffness of the subfloor to prevent flexing and subsequent tile/grout line cracking. The top of my keezer weighs a ton and if I didn't use plywood with a hardi-board foundation to tile onto, I would run the risk of cracking when lifting the front of the lid due to flex. I probably could have gotten away with 1/2" plywood and 1/4" hardi-board, but I would not consider going without hardi-board or cement board for this application IMHO. For the extra $10, mortar and screw down some hardi-board (sold at HD/Lowes in 3' x 5' sheets) and you should never have to worry about cracking. Good luck!