Years ago I dropped a bolt behind in the kick panel area on my car. I tied a magnet to the middle of a piece of string and pulled the string thru and the magnet grabbed the bolt and pulled it up after a few tries. No way was I going to leave it in there. Maybe you can feed a string from one of the drain slots up high?

If you have a farmers supply place around you can get a cow magnet and put it on the end of a wood dowel using a chunk of rubber hose. Cow magnets are narrow and can fit into tight spaces.