Sorry for the late reply. This came as a "visitor message" and not a regular private message and there was no notification. Not sure why there is a difference but if I hadn't gotten a private message yesterday I would have never known your message was there.
Yes, you will need a 357 barrel blank. Basically, just turn the front part down to where it will slip into the barrel and leave the back part shotgun shell diameter with a rim to locate it. The shotgun shell diameter part only needs to be about 2 inches long. Note that the back of the rim is at an angle and not square like a normal rim. I cut grooves for O rings to take up the difference and make it fit tight. Also, you will need a 357 chambering reamer.