You could easily write such an agreement, just take a standard model release, and put your intentions in plain English (model to receive ##% if all future sales.....). I did something like this when I got started and couldn't afford to pay my stock models, and didn't realize the can of worms I was exposing myself to (that fortunately stayed closed). What you have to do though, is be clear and define what kinds of sales count, and what don't count (all commercial sales? what about magazines where the model might not normally get paid but you do? What about sales if you sell prints in a gallery? what if you decide to give away something, for which you would normally get paid, does she get what you should have charged from you?), and also are you giving her a percentage of the gross (whatever the amount of the check is, or the net (are you deducting a percentage for your overhead first? Does the deal kick in before or after you recoup your expenses? Is she entitled to audit your books (as with most standard royalty agreements)? Does she have any control over whether to authorize the sale, or are you in control, and just having to pay her a royalty after the fact (never give the model veto power over the sale if you can avoid it, otherwise its what we call a "spec" shoot, not a "stock" shoot).
OK, now if all that isn't enough to boggle your mind, think about this: Who's responsibility is it to be sure they're in touch with the other, in case you lose contact and one of your moves away? How often are you supposed to pay her - upon receipt of payment from your client? Quarterly? What if the client doesn't pay and you have to sue. Will she wait for payment? Will she share in the proceeds of the lawsuit? What happens after two years - are you released from having to pay her anything, or is the model release canceled? When you say 2 years, does that include only deals you make within two years - what about deals you might make just before the two years, but get paid after?
OK, it sounds like a lot of details, but you should see some of the royalty agreements I get into these days as a photographer, and that's why I usually don't like them. If you find a simpler way to do it, great, but there's still one more I hurdle think you'll find. When you get out into the real world and start licensing images (we don't "sell" them, remember?) you may find that the fees are not as substantial in some cases as you think they might be, and the model's percentage (which I've seen run anywhere from 10-50%, depending on your overhead and production costs) will return a fee to her, that is substantially less than SHE thinks it should be. HER expectations of what is a reasonable fee amount, is often not realistic, and the problem arises when she discovers her 25% (for example) won't even pay her cell phone bill. This is where the friction can start. In my personal stock business, I do pay the models at the time of the shoot for a complete release, but I also will come back later and surprise some of them with a bonus if I make an especially large sale, or get a poster deal or something.
Now one more thing which will sound s little odd after all that. In a perfect world, they models SHOULD get a percentage of your sales. With computers and such, tracking and disbursing royalties is actually easy (although not worth setting up for the occaisional sale). The problem is, if you had to give away a percentage of all your sales for any reason, you'd be out of business. Many of the fees for stock use of pinup/glamour images are not that great, and giving away a slice off the top forever, would be a business killer, which is why I don't do it.
Andy Pearlman Studio