Silicone does not hold very well to acrylic. If you are gluing acrylic to acrylic, you should look for a cement like Weld-on. What kind of project is it for?
I'm working on putting one together myself (I'll let you know how it goes once I get started ). I am going to use WeldOn 4 for the joints and WeldOn 16 in case I don't do a very good job. The 4 is very thin and should make for clean seams. The 16 is thicker and more sloppy. Do you know what kind of acrylic you're going to use?
Yeah, WeldOn is the one to use. For small refug., it is not absolutely necessary to use casted acrylic, but it is always a better choice than the extruded one..
I get generec weld-on from my plastic store. It comes in a brown bottle and is labled "Methelene Chloride" it's less that $2 for 8 x or so. I still have the first bottle I ever bought.