I am often asked if house insulation foam can be used on boats. A few years ago I would have declared “never”. Since I have been building my own SIP panels on the lunar lander project, I have moderated a bit. But only on non structural items. I used the Foamular pink foam to vacuum bag panels. http://www.foamular.com/foam/
It ranges in density from 15 psi compression strength up to 40 psi or even 60 psi compression strength. I assume the shear strength is similar. I don’t see weights listed but it is light. One downside is there are no thicknesses of less than 1″ available. It can be gotten at Home Depot or any other similar store. If vacuum bagged with thin plywood faces, (3mm or even 2mm) it can make very light and inexpensive cored flats for your multi interior.  The edges will be much easier to smooth than any of the honeycomb or Nida products.