A piece of chicken wire of floral netting has advantage over floral foam:  it’s better at holding thicker stems horizontally.
 
It’s reusable. And it’s easy to camouflage, so no problem if it peeks above the rim.
Crumple a piece into a ball (with edges facing in) and hold it in place with two stripes of florists tape.
 
Size of the roll: 1m*25 cm