SandLock offers a cover for every model we make. Our covers are made from a Heavy duty Coverlight fabric and sewn to fit each model precisely. With our sandtables we sew a 5/16 shock cord into the bottom perimeter of the cover which allow the cover to be stretched over the entire table [...]

dimensions of a rectangular sandbox if the length is to be 3ft longer than the width?

Try some white vinegar, I know I use that in a house. It is not toxic, to your children obviously. Spray around the perimeter, and on top. I don’t know how if they are burrowing into the sand itself, it will take a lot of digging to find them all.