Form material for interior topping slab

Placing a ~3.5" topping slab to elevate a garage floor up to the house's sl ab grade on a remodel. We cut 6" wide strips of 1/8" Masonite to act as a perimeter form to set our screed lines. Wall studs are 16" on center. As I was transferring the grade elevation onto the form, I was wondering if thi s form material is sufficiently thick for the job without causing too much bowing of the form between the studs as the slab is worked and finished? T hanks for the advise.
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replying to kcmcgreevy, Iggy wrote: I wouldn't use anything less than 1/4" plywood...doesn't your Masonite just snap right apart, instead of having to be repeatedly broken and worked-apart? You can double-up your Masonite, but you'll need to glue it to get any decent strength increase effect.
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