Apparently one of the few things you can do.
If you think flush mount means there is no space behind the panel, try removing a flush mounted manhole cover and then step into where it was.
Already said that sandbags don't just come in big sizes but I guess you just don't have much experience with them.
If you think using RTV results in the use of GLOBS of it, then I guess if you have resealed either a pan or intake plenum your engine is a mess.
Already had said I was exploring using a different method to mount the panel but I guess you have reading difficulties and illogical thought jumps.
I may be a douche but I'd rather be that than a DUMB DOUCHE.