Fab keeps the stock metal tube up top so there's really not a lot of upper HG ventilation on that. You can drill holes through that metal part, etc, to allow more air to flow through.
The VZ 58 has two ports in gas tube immediately in front of the upper handguard. However, much of the piston gas vents under that front handguard. Since it's so enclosed the heat builds up under rapid fire.
That heat is also why the Czech's switched from wood to beaver barf as well. Wood always has moisture and the upper handguards get so hot so quickly that that moisture pushes away from heat rapidly, eventually steaming/boiling, and ultimately cracks the wood...
Best value all things considered is probably bonesteel. Only bonesteel and NEA/Troy cowitness w/ irons. All other foregrips do not.