Sandbox for concrete patio

We have concrete patio and currently we have plastic sand box there. However, this sand box traps moisture inside, and now that rain are over (I am in north California) sand is very wet and simply discussing.
I am thinking about building new wooden sandbox for kids, but I am wondering hat to do with moisture. Draining down is not an option due to concrete. I am thinking about have "vents" in the cover so moisture will escape, but I am not quite sure what exactly I need to do and I don't want to have caver too heavy.
Any ideas/recommendations?
Thanks,
Vadim
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Just keep the plastic box. Dump the sand, drill drainage holes in the bottom, line with landscaping cloth, refill with sand. If you really want a wooden box, do the same thing.
-- ******** Bill Pounds http://www.billpounds.com

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Drill/make drain holes around the bottom edge of the sandbox, line the sandbox bottom with window screening to keep the sand in and let the drainage run out onto the concrete.
John
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