i made airball bunkers for paintball with .5 mil plastic sheets from home depot, i sealed it with hot glue sticks and my girlfriends old hair straitiner it held air like a ballon, so im sure the process would work for a rain cover.
if i was going to make one i would build a cubed/ rectangular frame out of pvc that stakes in the ground, then fashion a plastic cover as described above that fits snugly over the frame, any ventilation holes could be easily installed and covered, i wouldnt think there would be any issues
ps if you make airball bunkers cover them with sheets so paintballs bounce off of them