Looking awesome !
I would thing a industrial grade adhesive would work like Loctite 505. Amazon has it : https://www.amazon.com/dp/B000BOCAU...af4-9003-96f70c48a63e&hsa_cr_id=5913377600001
I am assuming you will be blending the edges of Han into the surface of your backing board too. Do you know what you will use for that, or do you just plan on using Bondo ?
I've experienced working with expanding foam, that it is easier to scrape away the excess while it is still expanding than to wait for it to set up and have to cut all the access afterwards !!
Looking Good 5/0 !!