i think you could fashion a muffler.
you need a center baffle, just like in the links pic. chicken wire would be good a small gauge.
for the actual noise reduction i would go with something like cotton wool or toy teddy stuffing. something that would draw the sound so stuffing/cotton wadding would maybe work. motorcycle exhaust baffles (old type) used this kinda method, when i was young i used too burn them out.
maybe worth looking into a homebuild than shelling out your cash right now.
Tamzi