Many saddle bags come with a key clip on a short lanyard/tether. You could add a clip attached to a loop of cord, then sew it into the trunk bag. Or put the loop through any internal zipper pull, etc.
Or, the expensive high tech method
Tile uses bluetooth radio to make the fob beep when you are within 100 feet or so. Of course, you need to keep it's battery charged, have the phone app running, etc.
Remember where you last had it.
Always keep the app running in the background, and Tile will automatically remember the last time and place you last had your item so you know where to start looking.