Choate makes a half-decent side folder, with a metal folding arm, but if you ask me it looks stupid. Ramline makes a "waffle" patter synthetic side folder, but again, it looks stupid. I've got an ATI folder in the mail as we speak, and I'm planning on screwing & epoxying a weaver rail to the bottom of the forearm and installing a vertical grip. I'll try to get my hands on a digital camera so I can post pictures (I need to take pics of the range test for my reciever cover scope mount too, when I get a chance to head to the range).
Other than that, your choices are limited when it comes to mass-produced aftermarket products. If I were dead-set on an underfolder myself, I'd probably build something out of urethane foam/fiberglass composite. Urethane foam is nice 'n soft, easy to shape and sand and smooth into curves, and when you're done you hand-lay fiberglass over it and it forms a super-strong composite (like a surfboard, kinda). This would also allow you to adapt an overfolding stock from a shotgun or something, since you can customize your mounting points by putting threaded inserts wherever you want on the stock. It's a lotta work though, so I probably won't be doing anything like that myself anytime soon
Y'know what I think would be cool for an SKS is a full-metal (aluminum) chassis with an extend-o buttstock, kinda like that RB Precision stock for 10/22's (I think it's called the Evolution? Don't remember, look it up). I think a metal chassis would help with accuracy as well, as long as it fits properly and there's minimal movement. Just thinking out loud...