I don't know if either of these is "foldable." But they do work pretty well.
Available on Amazon. Claims it will fit on all Glock Models. I do not know if it will fit the Gen 1's though. I would call them and ask.
http://www.endotactical.com/products.html
I've used this one on a G18. Works okay, has a little flex. I like the "two point" one a little better as far as design goes.
http://www.zahal.org/products/tactic...o-941-handguns