Well - much fun is awaiting for an intense workshop!! I found some images of concrete benches - and some made without concrete that might work as inspiration.
There's a point to make with regards to the choice of materials: namely, where and how is precision desired and obtained via clamps or the design of restraining devises - and where is the fabric allowed to work its ways and deflect under the load of concrete. - Another question is how to deal with the transition of the formwork to the uncovered surface.
Light weight structures
[Image of pleat table from Arktura ]
[Coral bench by Arkitura]Above images are all but heavy - and they are not cast in concrete but by laser cut metal. A concrete mind would imagine these these algorithmically generated patterns cast in high strength fibre reinforced concrete - and soon after scale the fellow to something a lot larger - Otherwise you might use a big cookie cutter and stamp your flat sheet of Eternit (fibre reinforced concrete mostly used for cladding) and make a perforated version of the award winning 'Dune' concrete chair below by Rainer Mutsch.
Post to be continued - now
This post was actually much more elaborated but my computer is having an off day and deleted most of it (sigh) - so - this post continues right away for some examples cast in fabrics - click here when ready.