The gerber ridge and mini-ridge ($20 and $18) are good knives for an altoids tin, since they're nice and flat with the clip and thumbstud removed. That's still probably more than I'd want to spend if I had to buy half a dozen of them. If you could find a cheap knock-off, you'd be set.

Knifezilla.com has a few things that might be close, judging from the specs they list, but I've never seen any of them. Their $8 KISS half folder is a ridge-style knife, but it has a 3" blade compared to the ridge's 1.75". It's probably just slightly too large to fit the diagonal, but you might be able to "fix" it with a hacksaw and file, taking off the rounded end with the lanyard hole. Even if you cut it down, a 3" blade may take up more space than you want to give up. The $8 keychain CQC7 with a partially-serrated 2.1" blade looks like a better candidate. Or the $7 thumb fighter. Shipping is going to cost you another $6, so it still gets expensive if you want to order one, see how it fits, and then order the rest.

For one you might be able to look at locally, there's the gerber mini-paraframe with a 2.25" blade at bestknives.com for $10. I've only seen the full-sized one and it's fairly thin, but that's still a lot of space to devote to a knife. Maybe you could pack the holes in the handle with tinder or something.

I don't know if any of those knives would work for me; I'd have to make some hard decisions if I went with anything larger than the ridge.