I'm a very lucky girl and got two plastic jump blocks for Christmas! Cos they have a hole in for the moldings they've already got rain water in them... how do you get it out! I don't want them to get too heavy?
they blow over with just a puff of wind so best to keep some water in for weight, I found with mine even putting heavy stones on them did not stop them blowing over when the wind blew
I wouldnt worry about a bit of water, they are highly unlikley to fill up too much. If you realy dont want water in them get some mastic or other waterproof sealant (kitchen, bathroom, windown sealants) and fill/cover the hole.
turn them so the hole is at the bottom and they will drain! so where it got in it will get out if positioned correctly. once empty bung the hole up! but a bit of water in them will weigh them down come the windy season ...........