You can always mount it upside down, works quite well generally.
Until it fills up with water. My apologies, I thought it was painfully obvious that you need to add a rubber boot to feed the coax in from the bottom (now the top) and drill a weep hole in the top (now the bottom) for condensation and draining. Failure to do this will of course fill the radome with water. We've done this with a Telewave and a Sinclair, not tried a Celwave, but I suspect it's quite similar. Good luck!
