If you can, rough up both surfaces before gluing with C/A. I usually score a cross-hatch in both pieces with a hobby knife.
I use a 50/50 mix of Barge Cement and MEK. Apply the mix to the metal part and allow a minute or two to set. Place the metal part on the styrene part and, using a needle applicator, flow a small amount of MEK around the edges. The MEK dissolves the styrene and Barge Cement, bonding them together. Been doing this about 20 years now and have not had a part fall off or loosen. BTW, check out my forum, The Diesel Detailer (dieseldetailer.proboards.com).
