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).