I'd suggest Creative Paperclay for this project but many brands of ADC should be suitable, including the inexpensive Crayola Air Dry Clay or the heavy DAS terra cotta clay. The really soft clays probably won't retain a stamped impression very well.
Remember, all air dry clay will shrink some, so be sure to remove from glass armature to dry or you may find a lot of cracks in cured clay.
Keep in mind that, even when cured, air dry clay is water soluble...so you must
use the test tubes to hold water as shown in tutorial or another type of glass insert. Of course, artificial flowers won't need any water! ;-) LOL
Inspiration and ideas for other styles: