To deal with the tension and compression, the spar cap will take the majority of the load. Foam core/honeycomb are to prevent buckling. Expanding foam core is really nice. It's not heavy. I want to say we used the 10 on our 150 pound plane. 5 might work for you and it's lighter. Just don't blow the spar...

https://www.amazon.com/Foam-iT-5-Rigid-Polyurethane-Foam/dp/B00O5FBQJK?th=1