What should I use for a sandbox bottom?

Just thinking off the top of my head maybe a wooden (cypress) frame, put a metal grid across the bottom (maybe some kind of galvanized grid) and brace it with some pieces of cypress. Then put the landscape fabric over the metal grid and then fill with sand.

I almost said go with a straight metal grid but the weight of sand and children would probably be too much so the pieces of cypress would brace across the metal grid. This way you'd get good drainage and the ability to move it around (if you put it on casters).

Not sure how resistant redwood plywood is to rot or if they make cypress plywood.

Since you are going to place it on a deck, move it around, want to keep water and cats out, I would just buy a plastic one with a lid. In my opinion its not worth the hassle. The wood alone will make it heavy and hard to move around, let alone with sand in it.

They are fairly cheap, we bought one a few weeks ago at a garage sale for $5. I know there is pride in doing it yourself, I wanted to build one too, but in the end, it may not be worth it.

I used a linoleum remnant for my kids and it lasted for 20 years. Now I can't find a remnant anywhere for my grand kids' sandbox. The linoleum worked great as it could still drain out the bottom and it kept the sand clean as they did not scrape up dirt when building their "roads" in the sand. I thought of putting outdoor remnant over the top of the sand as they could lift it and the cats and squirrels couldn't dig in it.

Use a tarp with lots of tiny holes poked through it. I used a meat tenderizer to poke the holes through. I simply cut it to size and stapled it to the bottom. Works great :)

Landscape fabric because even if you was to put wood which would rot. Once sand was put in the weight would keep you from being able to move. Landscape fabric always easy to get and replace. Here is a great design to keep critters out and let the rain drain thru. Jae

I thought of these good ideas but In my case the sand boxed area is enclosed outdoors and the ground is covered with pea gravel. Instead of removing the gravel I kept it as it has a cushioning effect for the knees and drains water thru to the soil beneath. After much thought I decided on the exact size piece of a well woven rug remanant BUT turned it upside down as the threads were so tightly secured only minimal sand can seep through. Then poured in 7 bags of white sand. This is working great so far. Should moisture cause mold I'll bleach the entire sand box. It appears that the layer of pea gravel causes the carpet and sand to stay somewhat aerated. So far my twin grandsons are loving it and are even able to add a "little" water without causing a big mess! So what do you think? Just me, Lisa