You should be using a product called Weld on. There are different varieties, but I think #4 is probably(someone correct me if Im wrong) the one you want. You can get it at the plastic suppliers listed above, and you should be able to get a good size can of it and applicator for about $10. You'll want to make sure you have nice smooth level edges also, becuase you will be bonding the acrylic together, not just glueing and sealing

BTW, as you've found out, silicon doesnt stick to acrylic too well, and is also not good to use on PVC...cs