As a concrete professional I will offer these suggestions: The best option is buy styrofoam from Lowe's or Home Depot.(They offer various thicknesses in 4x8 sheets. Stake it in with stakes and tiewire or stringline etc. If it ever has to be removed just pour some gas over it and it will break down to nothing.
You can always use wood and wood stakes. Just build your perimeter form and fill it in with compacted dirt or sand. This is called soffit fill.(or put a wood top on your form)
Anything else would be too expensive and I build condos, bridges, flood channels and I have also poured Stallone's driveway and work at the studios and other star's homes.. I have pouring and running Civil/ Public Works projects for 11 years from Florida to Illinois to LA and Las Vegas. (PS I learned in the field not from a book in a college class) I know what works not what looks good on paper, or what a book says.