Carts have been discussed at length on other threads. You may want to check those out to see what our current situation is and why the modders have hit a wall, so to speak.
I like the metal cart idea though, but I think the reason they're currently made out of plastic is because:
A: Reduces weight of the device (arguably negligible)
B: Cuts back on costs for the manufacturers (imagine the cost of molding simple plastic as opposed to having to pay to cast or fabricate metal cylinders, now multiply that by about 400k)
C: Losing plastic carts is no biggie (and yes, they are easy to lose)
D: They are disposable (if it gets nasty or chewed up, throw it away!), making it more cost effective for the suppliers, especially if you only buy pre-filled
These could probably be solved by making them in small batches by distributors of e-cigs (or sold by modders). You would have to find a metal that doesn't react with the oils, water, and alcohol-derivatives in equid though (I was thinking about this very same thing just yesterday). Maybe coating the inside with teflon?

I like the part about coating the outside with copper though, but you have to remember that some people like to bite down on their e-cigs though