stalagmite technique? could vary the size, but tissue paper is so much cheaper than the spray foam. Not necessarily flowers but something else in giant size? - Giant Tissue Flower | Tutorial (some steps)
DIY - Tissue Paper Flowers ( I love these <3 I made these but much bigger to hang from the ceiling of our wedding reception =) They looked great & the lights bounced off the colored tissue paper which made it just beautiful throughout our venue!