the files usually have supports made of paper to keep the shapes - ive no idea what smoothcast is, but the best way for pep files is poly resin. brush on a layer to the outside, allow it to dry and repeat 5 times - this will sturdy the paper bit by bit and when it comes to the sanding process you will be rubbing down the resin and not going through into the paper giving you loys to play with. when the layers are all done then work on the inside - a nice coat of resin on a section, then add the matting and resing over the matting. cut some matting in squares or rectangles and simply put it in position and then resin on top. it will be hard withing 2 hours and really solide after 12 hours
this is what you will get after 5 outer resin layers and the inside matted - harder than john holnes when dry


of course it will be solid and wont bend - so if it doesnt fit simply get a heat gun on it to soften the glass and bend it to the shape you need to fit the body - when its cooled it is hard again. just make care when doing the hard to access bits.... small bits of matting in intricate areas so you wont end up rubbing through into air pockets / unresined matting
this is as far as i took my pep build - mainly due to the uncomfyness at this stage, esp as i have back issues - i found the fibreglass to make a person too restrictive

and a pic of the pep paper supports that were on my files:
