I used USC all-metal its a aluminum based body filler. Its very strong and a pain to sand but most importantly its water proof were most other standard fillers are not.
USC Dura-glass is another one that can be used it is a fiberglass based filler that is also water proof. I have used both before, both seam to work well.
The dura glass is nothing like you ole fiber glass stranded tigerhair, it spreads like a standard bodyfiller as dose the all metal both are a heavy filler.
I just use either of these as a base then use what ever filler over them as both duraglass and all metal are a pain to get feathered out.
also there are others that make the same type of fillers.
Thanks for looking