I used an mock suppressor, bored out one end of it so it would slip over the barrel, and then put it on backwards so it covers the barrel, got the can off some random airsoft site, just had to measure the length from the front of the gas block to the end of the barrel.
Cost varies depending on how much your smith would charge you to cut and tig weld the front sight onto the gas block, or if you want you can try to source an FSN01K gas block with the front sight built in, i had sourced out 3 originally, it was quite a lot of work digging the internets for them!
ended up getting them out of the Czech republic if i remember right... it was a few years ago tho.
easiest way to replicate this would be to cut and weld the front sight, as its cheaper and much less hassle than trying to source an FSN01K gas block