if heat is not a problem for you. a small hurricane lamp w/ rubbing alchohol works.
I found a formula in a growbook yrs ago to determine how much you need. dont remember the formula but for a 4x4x7 box,1 oz burned over 10 hrs was enough to bring CO levels over 1200 ppm.being my box was not airtight & still had a small exhaust fan running,I used 2 oz of fuel- never metered it to confirm actual levels but it helped alot.