I used a "sharp shooter" shovel to cut the foam from the stringer edges, then into ~6" square blocks, then pry the foam out in sections - kinda like carving brownies outta a pan. A 4" putty knife made a good scraper. Messy, tedious work for sure, but looks like you're making good progress.
rlhahn - the loading on the platform brackets is vertical, so you're better off making the backing plates taller instead of wider. I really like how Okie's go all the way up and capture the tow rings as well. Here's a shot from his thread that I saved cause I'd like to do the same...