What I can use to waterproof that wooden AC tray?

I'm looking for a good suggestion. I have my air conditioner sitting in a wooden tray, but I don't want the wood to get wet and rot out.
It's currently covered by a couple layers of spar urethane, but I don't think that's enough.
Any suggestions what I can use to waterproof that wooden tray?
Mike...
It's currently covered by a couple layers of spar urethane, but I don't think that's enough.
Any suggestions what I can use to waterproof that wooden tray?

Mike...