I was looking at one of the roadblocker barrels up close yesterday.
My local shop had one on the wall and I couldn't resist an up-close look. The barrel itself tapers/flares out just before the brake, so it would appear that they use the heavy breacher barrels and pin the brake onto the breacher tip. That's just an observation, but the breacher barrels are the only ones I've seen with the flared muzzle. If that is the case, you might have trouble finding a way to attach that brake to a standard barrel. I would say that @
MikeD 's suggestion of simply picking up a roadblocker barrel is your best bet. The barrels usually run around $150. By the time you buy the brake and have it threaded or pinned onto a barrel you'd be spending much more than that on parts and labor...