Henry...the insulation -foam is polyurethane.it is homogenous and of even texture throughout....Similar stuff is used in foam-sandwich construction for boats,light aircraft..... It'sa doddle to chopand carve.....gondolas can be attached to piano-wire stays that can be simply poked intothe foam,withdrawn, smeared with glue and refitted.
If Mrs Crun is inclined to flower-arranging (other than the D P Gumby variety!) she willhave had experience of "oasis" a foam block which the stems are poked into.....but somewhat dearer than the insulation offcuts.
Some of this insulation is about 6" thick, so you could just use an appropriate-size of tin like a cookie -cutter,maybe,then glue the stack together.....OTOH,the stuff is foil-skinned on at least one side, so selecting the right thickness for your discs would give the "hoops"....longitudinal grooves could accommodate wire or wood "stringers" and a quick rub-over with coarse sandpaper would erode the soft foam"panels" pretty good, I imagine.
In other words, like wot GQ2 said!