or make it hollow in the middle?

of course: a big container (15 cm x 8 x

could cost more than 20 euros only for the resin if it is a one piece.
The point is that you still need six faces, each 3-4 mm thick, it means 4 euro of resin, plus the work of casting, and you still will have to assemble them!
Result: quite an heavy duty, and the price will be quite high.