It's super cheap, and super easy, and works perfectly. Here are the steps:
A vinyl table cloth (size to fit-a 70 inch round works perfect for our sandbox). This one was $3.99 at the local Walmart)
a sewing machine
One safety pin
Take your vinyl tablecloth out of the package and lay it out on a flat surface. Fold about one inch of the outer edge in, towards the underside, and secure it with straight pins. Do this all the way around.
Set up your sewing machine, and sew a simple stitch all the way around the entire outer edge. The only thing to remember is to only sew the edge of the folded tablecloth because you need to leave a hollow space along the perimeter large enough to string your wide elastic through. (this will all be clearer the more we go through the instructions-it really is EASY).
Take your large elastic and put a jumbo safety pin in one end of the elastic. This will help you string the elastic through the entire edge of the tablecloth. It will also help you find the end of the elastic if you ever loose it in the process inside the tablecloth edge. I also had to buy a few packs of the wide elastic and sew 2 of them together to be long enough. Not a big deal.
This is the slit that you should have cut into the outer edge of the tablecloth. This slit will be the opening where you insert the wide elastic. I always attach a gumbo safety pin to one end of the elastic. This helps me to feel and lead the elastic through the entire edge of the tablecloth and also helps me find it if I lose my place. You'll be doing this 'blind' since you cannot see the elastic and safety pin while feeding it through the edge of the tablecloth. When you have fed the elastic all the way around the perimeter of the tablecloth and have come full circle to the slit that you cut; simply pull the 2 ends of the elastic and tie a knot to secure all your work.
When you are done you'll have what looks like a giant shower cap:)
Here is the top view.
Viola! That's it. Super easy, simple, cheap, and it takes about 30 minutes to do from start to finish.
Here is our sandbox
And...tada! Here it is with the waterproof sandbox cover on it.