Those plastic things probably cover crimped metal ferrules. You might think about a mid-range chain + shrouded padlock if weight weren't a problem (it would be more secure). Like this one:
http://bicyclesource.us/itemdetails....andasc&id=7791
If you could leave the lock at the destination, then the weight would be a non-issue. And if you can lock to something solid with both the U-lock at the rear, and this one at the front, then that's double trouble for thieves.