the softies i have used were turnigy 3000 mah 2s so they are thin. i use a foam block to keep the down.
i have also fitted 3000mah 3s in there, its about the same size as a 5000mah 2s hard case.

no mods needed to fit them in my b4.

keep in mind some tracks will not allow you to use soft packs at all.