I haven't seen a thread on it, but take a look at the Aqua C collection cups. The premise is basically a ping pong ball in a tube. Keep in mind that there's no reason it needs to be round, so if you can find an appropriate container, you can probably craft the rest cheaply from PVC.
In theory, the ping pong ball (it's probably not actually a ping pong ball, but it's essentially the same thing and one can be used as such) will float up, cutting off the air going into the skimmer when the cup is full.
I'm thinking a sealable container (if a substantial amount of air gets in it won't work), a few pieces of PVC, a ping pong ball and a hose barb to attach your skimmer output should do it. If you get a little creative, I think you can do it.