Rather than using straps to hold the form together, I reinforced the corners with some scrap, to keep the nails from pulling out. It worked like a charm.
I tried to keep the concrete from getting too soupy, so it really didn’t exert much sideways force at all.
If I had to do it over again, I would have used 2×4 forms, instead of 2×8. Pour the concrete, screed, float. Then either piggyback another set of 2×4 on top for the perlcrete, or wait for the slab to set and then move the forms up.
I decided to rest the forms on top of the concrete blocks to avoid building supports for them. Hopefully the slab still has enough purchase on the blocks to keep from caving in! One thing I noticed from this photo is that I moved the form when hammering in the reinforcements, and forgot to nudge it back.