Shims?
Do you mean walls to create a divider wall?
If so, nothing.
Just put the plastipaste on and move it into position. Then as the materials starts to kick (you will see it start to get cloudy and cream color) dip your gloved hand into rubbing alcohol and use your fingers to pinch the material into a wall. Also while you are at it, smooth the surface of the plastipaste to reduce those annoying finger stickers. Remember to use paste wax as a release between sections of the support shell. Be liberal with it. The imperfections in the wall that you create will acts as keys for you, or you can add keys into the walls. Pinch flats where you want to put bolts to hold sections together.
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