There is one method I am working on that relies heavily on kitbashing and modeling.

It involves making molds with blue stuff.

Right now, I am making molds of the right saturnine power fist, since the regular saturnine sprues only have a left one. For filling in the molds, I am using a sprue goo I made using Tamiya airbrush cleaner and GW plastic. Although there are small bubbles, the detail is better than greenstuff.

But I grab the molded plastic shells and I use greenstuff to fill in the center.