increase the number of the walls until it fills the model.

You can usually fiddle around with infill enough to make anything solid.

You can also load the stl into meshmixer and make it solid.

Likewise windows builder is actually useful for that kind of thing.
And as a 30 year computer professional, I can count on one hand the number of times I've ever recommended a microsoft program. But builder has it's uses.
It's particularly good at adding text to an existing model. Even flows it round corners.