A couple options for ya:
Get a telescoping magnet (hear me out!) and wrap the end of it with duct tape inside out (so it's sticky) - then use that to fish out the clips. Another option is the flexible claw grabbers. Since those don't work for crap on parallel grabs, you'd do the same thing with the duct tape. Plain old 12ga house wire might work too (just something kinda rigid to get the tape ball down in there).
Second approach is just confirming that you can use expanding foam with no real dangers. I used to use the stuff to fill cavities in competition stereo cars all the time (probably a dozen of my installs used it - including 3 of my personal cars). But, the problem with the regular "great stuff" is that it will expand harder than sheet metal can withstand if you put too much in a cavity (only time that happened to me was on my wrangler's tailgate and fortunately the tire obstructed it - sucker was SOLID afterwards though ). The solution is to use their product intended for window and door jambs. That stuff doesn't expand with any force, so it won't bloat your panels if you get too much in there.