For what it's worth, the lugs on my GPR appear to be spot welded to the barrel.
This would explain why Lyman doesn't list the underlug as a purchasable item in their parts list.
http://www.lymanproducts.com/store/page155.html
The size of the existing underlug on my rifle is .313" wide X .200" high X 1.330" long.
There is a slot passing thru one side of this which is .590 wide which is .135 deep leaving about a .065 wall. This essentially makes a open sided upside down "U" shape with the barrel flat forming the closing side of the U.
Track of the Wolf doesn't seem to have a replacement underlug for this rifle and the ones they do offer don't look like they would do the job.
A piece of 5/16" X 1/4" X 1 5/16" stock could be used to make a new lug but I don't recommend trying to spot weld it in place.
I suppose both legs of the U could have a .050 high X 1/4 wide dovetails milled on them with corresponding dovetails machined in the bottom flat of the barrel to attach it.
The other method would be to take the existing underlug and file the two interfacing surfaces flat.
Then, lightly filing the bottom barrel flat to remove any projecting material and the bluing a good low temperature silver bearing solder could be used to attach it.
On the positive side of this problem, the rear barrel wedge isn't absolutely necessary for the rifle to operate.
The forward underlug should be enough to hold things together. After all, there are thousands of half stock muzzleloaders that only have one forward barrel wedge in them.