Very 'tricky/unnatural' syntax for this 'idiom'
le gâteau était en forme de fusée ! so forme and fusée are two nouns with noun 2 acting as adjective (like 'la salle de classe). No article before first noun because of preposition 'en' which kinda fools us(well me) a bit. I tried "they made rocket shaped cakes" and got "ils ont fait des gâteaux en forme de fusée". Rockets have ONE SHAPE and thus all the cakes had that ONE shape.
I tried "they made cards in the shapes of flowers" and got "ils ont fait des cartes en forme de fleurs". Not 'formes'! Usually even behind idioms are solid grammatical truths.. I suspect this is a dumb question but does the use of 'en' in this context require a singular noun. Examples of 'not' dont come to mind... unless with a noun like 'larmes' which is really be default plural.