You could try fitting a stadium truck or buggy/truggy body to it so the tires are outside the body. That will keep the majority of the sand out of the tub.
Not as cool looking imo, but function trumps form sometimes!
I use these on my 3s senton and they help keep a lot of stuff out if the chassis tub, but not sure if it would help you in your situation or not
https://scorchedparts.co.uk/collections/traxxas-slash