I would not worry too much about this if I was you. Any looseness that develops in the panel in the door will be barely noticeable, as the weight of the panel will push down on the floor of the groove and will resist movement. If you truly want to avoid any looseness, but still allow the panel to expand and contract, then a little glue, in the middle of the bottom edge of the panel would be one solution. Personally, I wouldn’t bother though. To fit the bevel to the groove, make a small gauge block with the same groove width and depth and use that to check the panel, rather than assembling the door each time. If you do make the panel too loose, then you can also wedge the back of the panel in the bottom rail with a few plane shavings once the glue is dry, trim them with a knife and they will be almost invisible. This will allow the panel to expand and contract still.
Colin, Czech Rep.