Alternatives to springs for recessed can light trims?
Welcome to the world of recessed lighting. I've run into this a few times and have used common springs. You have to shape the ends to fit into the can. Rubber bands and elastic string won't stand up to the heat long term. 

Instead, exact replacements are available online at $1.18 for two.

Since you're installing 40 trims and need two "springs" per trim, any retrofits will take a lot more time to install. It might be worth the price to buy the springs and do it right. Good luck.