Formwork is the unfortunate, essential-yet-absent material that can 'make or break' the concrete pour (or the budget, for many architectural projects!) I'm happy to say that my formwork did its job well- but sadly, it is now waste.
I chose to make the frame out of MDF, knowing that it would not be salvageable after use. The inside is thin plywood, in order to follow the curvature.
There are, of course, different kinds of formwork that are reusable, but for a small, unique slab like this, its life ends with removal.