I would use some thick acrylic cement ( stuf used to make acrylic aquariums ) for the sand and some epoxy ( standard buy anywhere stuff ) or a silicone without the mildew inhibitor..... let it cure outside for a day or so and wash it well.... should be fine ... just to play it safe put some carbon in the aquarium.....
I have used the acrylic cement with no issues many times when building flow diverters or stands in the sump.... just let the solvent evaporate , thus letting the glue dry and then give it a rinse with some tap water...
The acrylic cement " melts " the acrylic thus letting you put a fine layer of sand onto it. Might want to look into some clear two part epoxy paint also.. guys used to make wood aquariums then paint the inside with epoxy paint, the paint generaly comes as a clear and then you add pigments to get the color you want.give the acylic a good sanding to rough up the surface if you go the epoxy route.
The place I picked up the arcylic cement was johnston industrial plastics in edmonton..
http://www.johnstonplastics.com/