Barge Cement
The only adhesive I use for metal-plastic bonding is Barge Cement. It's a rubber type cement often used by cobblers for repairing shoes. It can be a little hard to find though. Ace Hardware seems to be the only major hardware chain that carries it, however they tend to stock it erratically. I've had good luck finding it at higher end shoe stores, and also any shoe repair business should have it. Then of course there's the internet where enough searching will find anything
Barge is a little thick, so it's best to get a glass jar (of the sort you would use to mix paint with) and thin the Barge with Methyl Ethyl Ketone (MEK) which is what I use, or a Lacquer thinner. MEK can be found most anywhere... I think I got mine at Home Depot. Anyway, dilute the Barge 50/50 with your thinner of choice and use a small cheap paint brush to apply it to whatever you want to adhere. The nice thing about the thinner is that it melts the plastic slightly, giving you a bit of the plastic "weld" that you get with straight up plastic cement. Check your mixture regularly though, I've found that under certain climate conditions the thinner tends to evaporate leaving you with straight barge again. Just add some more thinner though and you're good to go.
I generally apply the mixture to one surface and then attach, but some people recommend applying it to both surfaces and allowing it to dry, then lay down the part you're attaching, and brush straight thinner onto the joint, the thinner will be sucked into the joint by capillary action and will reactivate the Barge, sealing the bond very tightly (this method is particularly effective for laying down brass treadplate). For bonding larger or heavier parts (such as weights) straight barge (undiluted) works quite well.
Barge has a lot of advantages over CA and other options. For one thing it doesn't dry instantly, so you have the ability to position your part before it sets. It also won't get brittle, or cause parts to pop off as CA will over time and with exposure to air. It also won't craze the plastic.
Check out my blog:
http://milepost15.wordpress.com. The F unit I'm building was assembled using Plastruct cement for the plastic-plastic bonds and Barge/MEK for all the plastic-metal and metal-metal bonds.
