If cost is a factor, we charge around $30 to $50 depending on your front sight options. If you want an expensive front night sight add in that cost. You can do it at home with a pipe cutter, cold blue, and a file, but making it look good will be a lot harder.
Buy a second short (18.5") commercial barrel.
Agreed with others, Fed NFA rules deem 18" the minimum allowed without it becoming a Short Barrel Shotgun. As to being a sawed off shotgun that doesn't become in play until by definition the barrel is less than 18 inches long.
But also agreed with others, good luck in using that in a HD case and it becomes discovered that you used a 'sawed off shotgun' as the media will excoriate your good name. It's cheaper in the long run to buy a used or even new 18.5" barrel.
yeah, i ended up backordering one from Cabelas today. For some reason, everywhere I looked, it's out of stock. Hopefully I'll have it within a few weeks.