It’s important to work fast once the foam is in the hole...
I could have inserted some sort of J bolts into the wall as I was going up with the papercrete but I didn’t want to commit myself to their locations. As it turned out I decided to put a door in the north wall and moved the location of the kitchen which meant moving the window. I guess I know myself well enough by now to accept the fact that things will always change.
I cut some threaded rods to the length of the drill bit. The rods and bit are the same size. I drilled holes into the frames at several places and filled the holes in with expanding foam insulation.
It’s important to work fast once the foam is in the hole. As soon as it starts to expand stick the rod in the hole and bang on it with a hammer until it is flush with the frame.
After that I went all around the window with the foam filling in all the gaps.