I bought a bike that had a brand new set fitted, used them once and put on well used mitch front and IRC rear. Infinitely better, everything else on the bike was the same. (This was about 18 months ago)
Just a perforated tube (or should be, they get blocked) I cut a rectangular hole on the carb side, repack with proprietary material. Cut and bend a cover fixed with four rivenuts, sealed with high temp silicon.