I've tried the tri-rail Leapers mount, with some success, but it definetely makes cleaning a LOT more of a chore. Part of the beauty of the SKS is that you can have it in pieces on the coffee table in less than a minute

The best system I've found so far is the Kalinka quick-detach side mount, looks like this;
And the side rail you need to mount it looks like this;
Here's the breakdown;
P/N; AMAKPOSPSMWB, Low-profile Weaver Rail side-mount, $49.97US
P/N; AMUP, Universal AK/SKS/SVD side rail, $12.97US
Total; $62.94US
With shipping you're lookin' at around $75.00US
With this system, you have to cut out a relief in the stock for the side rail, and you do need to drill & tap. There's a detailed article on surplusrifle.com, regarding the side-rail mounting procedure.