Raised Garden Bed question? ?

I only have space at my home for a raised garden on an empty concrete slab measuring about 6ftx6ft. My question is what do I put on the bottom of my framed bed to allow for drainage on a concrete slab? I was planning on putting two pieces of plywood, raised just slightly at an angle to allow for minimal runoff but I'm worried about termites and quick deterioration of the plywood underneath such heavy soil. Any other suggestions for drainage on raised bed on concrete?