Styrene glue can be homemade from Methyl Ethyl Ketone (MEK), which is available in hardware stores (paint dept.) in the US for about $9 a quart. Put a few ounces in a glass jelly jar or a metal can with a screw lid, make a hole in the lid just big enough for a small brush to enter and screw the lid on tight (MEK evaporates very quickly if not contained). Make a tight fitting plug for the small hole. Cut some scrap styrene into tiny pieces and drop it in the jar. The MEK will dissolve the styrene. This gives it a bit of consistency and a small gap-filling ability. Apply it like someone else on this thread said, with a small brush. Fit the parts together and tape them in place before applying the cement, because it melts the styrene and sets in about 20 seconds. It is dry and hard in an hour or less.
This is not my idea. It came in the instructions for my Model Slipways Loyal fleet tender. I have built the whole boat with this stuff, where styrene-to-styrene joints are made.