try toilet paper ....

Or that foam they send lights in ..That 2 foot long sheet rolled up makes a light saber diffuser.. try coffee creamer bottles , smoothie bottles . Avoid most of the milk jug type materials add a kind of dull grey cast to the beam . same with your basic coffee cup lids..too much grey tint for me . Film canisters, medicine bottles < usually kill too many lumens .

I'm selling 15 different sheets of materials at like a $1-$1.50 a sheet

Hardly what I'd call expensive .One sheet diffuses about 30 convoy S2's .../

I'm not saying don't use odds and ends because I still have some odd materials on a few lights ...but upgrading to better diffuser isn't really extravagant :P