If you already have a key ring attached , possibly a hanger, duct tape, & some Macgyver skills !
Get a telescopic magnet tool is probably your best bet. (unless you can find a snake scope/camera to really see where it is) Try to get get as much as possible while fishing around in there.
When You here a ' clink" (magnet grabbing something , possibly your key) You want to have better advantage to spot it as your reeling it to you. slowly methodically. What you don't want to end up doing is, pushing it in further while feeding in the tool! Other wise its really going to be fishing in the dark. Best of luck. hope this helps.
If only had a flexible, telescopic, camera/ flashlight/+tools like magnet cheaper than your key
Or like i do