Have you tried that litter deodorizer? I sometimes use the Arm & Hammer "Fresh & Clean" litter box one and it works really well. You put it in the litter and mix it up. I add a bit each day, after I clean the boxes out.

Do you have a window in your room you can crack open a bit? We have all the litter boxes in our den and we always have that window open, at least a crack.