Totally agree with Kaine about the plaster being strong and not that heavy. However, if you really want to use resin, you can get some silicone, cast up one set of plaster and make a mold of the pieces you cast up. Then the silicone mold can be used to cast resin blocks. I've done this when making wall sections. I'll make one wall section out of plaster bricks, then make a mold out of silicone from Smooth On. Then I can cast up as many resin walls as I need to keep the weight down. it's not a cheap option. Resin costs quite a bit more than plaster and and silicone will be an investment as well. But, if weight is your concern....