I have had success holding heavier items like a small shelf unit to the foam core side walls using the following technique:
* drill 1/4" hole (for #8 screw anchor) approximately 3/4" deep in the wall where I want a screw
*
cut off the tip of the self drilling plastic drywall screw-in anchor so it is only left with the screw thread section
* inject silicone in the drilled hole
* screw in the modified anchor
* let the silicone cure for a day
* mount your item to the anchor
Here is the type of anchor I use:
