Think of it as a commission small enough to wear on a pin or something similar.
For example, I have some Last Resort Buttons I made a month ago - If I had done them with the character's names on them, and done them as commissions for other people, they'd be conbadges.
Of course, Some things work better than others. I love the Sculpture look, for example, but they're tricky to get/make/send. Lapel Buttons work (and I make 'em), but they're sorta small by badge standards and get lost in the 'look' pretty easily.
What works for you depends on what you can afford and what you're willing to wear in a crowd. For obvious reasons, you're not going to see a lot of conbadges with yiff on 'em (though I'm sure they exist), and while the fancier ones look better, they're also more likely to break.
You can either get a lot of variable quality, or you can invest in one you'll love, or you can make your own. Whatever you do, just pick something and run.