If I remember correctly there was a way to make your own surface skimmer for the Nanocube 12. Can anybody recall how to do this?

Bet you mean the old cassette case mod. Need the back of a cassette case, one thats a full back, not the other kind with the corners angled on it, cut some slots on the top, and just stick it over the intake grate. Water pressure hold it in place. Several aquarium supply sites DO sell the JBJ made surface skimmer though, usually inexpensively too. On marinedepot.com

Yeah that one! I'm probably going to buy a surface skimmer but does this mod work just as well?

Yes, all it really is is a thin plastic box with one big side missing and teeth/slots cut on the edge of the other. the JBJ manufactured ones (and ones like mine) are just prettier.