Those are some terrific mods. Probably the nicest modded Sherpa I've seen, and I've been looking at lots lately since I just bought one and I'm huntin for ideas.
I'd like to see more pix of how you attach the top box to the rear rack, or maybe you can explain it in more detail.
BTW, found a link to those surplus boxes, in case anybody's interested. Lots other cool stuff on their site, too: