Denture Tablets
I always keep a box of cheap denture tablets on hand.
Not only do I use them to periodically clean my mouth guard, I toss them into pots and pans with stuck-on food. Often, the tablets will help loosen the gunk.
Plus, they smell good, so sometimes I use them just to freshen up plastic ware.
One box lasts for years, so it’s a cheap cleaning investment.