Some years ago, Revell's (Germany) kit no. 5081 was the full-hull (no interior crap) version of their 1:144 scale Type XXI U-Boat. For a reasonable price, this kit was a good buy and a good build.
Thorough research clued me in on what detailings were needed to get this kit into an accurate model of a Type XXI, and the kit wasn't too far off to begin with. Cutting open the limber holes and countless other vents, access hatches, and doors is a standard procedure, in which case, I'd fashioned a false inner hull and various under-deck fittings to not leave it looking empty and hollow. But I had plans for using this model in a diorama, so I took a lot of extra steps to open things I wouldn't normally bother with.
Grab handles and railings made from thin brass wire were among added details, as were photo-etched louvers for ballast tank and intake ports. The model's hefty size made it easy to take further measures for realism.
Normally, with a stand-alone model, I do only some very minor, if any, weathering effects -- why put an old used car in the showroom window...? But since I was planning to put this model in a diorama setting, I gave 'er a good, thorough weathering to show a lot of seawater wear and tear. Toward the end of the war, Germany wasn't too concerned with proper primers or adequate anti-fouling coatings, so the hull paint on later boats didn't stand up well to deep salt water or to lingering alongside in endless tides.
With the recent fad in huge 1/72 scale models, I wouldn't be surprised to see this nice kit get retooled to larger scale. It would be a good one.