I would be carefull with the WD40. Probably just serve to collect dust in the long run.
Also be careful with degreasers and engine cleaners. Anything with MEK or toluene may damage plastic.
If it were me I would brush it out as much as possible and then use methanol or isopropanol. This will A) not eat or damage any plastics and B) will not leave any film behind.
Rust should not be a problem if you get it all clean.
Also maybe find a way to seal that up so you don't have to go through this often.

my two cents....