In my office I use a similar foam wedge with a
large "V" cutout. I shoved it into a "Tygan"* bag.
This is not quite as effective but more comfortable
for long-term sitting but I spend most of my day on
it so it can't be that bad!
*("Tygan" is the material used to make really
strong mailing envelopes. This one is a big mailing
bag that arrived with something my wife ordered
from a catalogue).