The receiver cover and the gas tube are the two least stable platforms for mounting an optic. Remember, even the slightest, most invisible movement of the optic will throw your shots into huge groups. Like the 30 round magazines, I have seen very few positive reviews of these mounts. There are a few solid mounting options though.
Choate and ATI make solid receiver mounts. The Chaote unit is the better choice in my opinion, as it lets you remove the receiver cover for cleaning, where the ATI unit needs to be removed first. This means reinstalling and rezeroing every time you clean the gun. The downside is that they both need a little work to install, including drilling/tapping holes and relieving a bit of material from your stock to allow room for the mount.
Millet and another company who's name eludes me at the moment make mounts that replace the rear sight. I have the Millett unit on the SKS I use for brush hunting and it works well, though unlike the other, better, unit, it needs removed to remove the gas tube for cleaning.
You can also attach a side rail to the gun, allowing you to use optics and mounts made for side rails. Kalinka Optics is a source for them. Tantal also has the rails and can order various optics to fit them.
Those Tech Sights are really amazing units. Combine one with the KNS crosshair front site and you'll wonder why you ever considered optics in the first place. That's as accurate as an iron sighted SKS is ever going to get.
