Coolers stabilize fast depending on the size, because coolers do not have cedar that helps regulate RH by absorbing/releasing moisture slowly over time. I would suggest a large credo or makeing you own humidifier. Because there is no cedar, the humidifier has to do all the work.
As far as sealing goes, go to the nearest marina and get sail makers wax...Now for all you land lubbers who don't know what a marina is...Never mind, would take to long to "splain it to ya". Take the wax and cut out a flat piece. Next cover your 'dor so nothing can fall in. Now with the flat side of wax you cut out(at least an 1 1/2 inch thick), lightly rub it on the lip of the 'dor. Long strokes from one end to the other as to apply as evenly as you can. Only one layer at a time(close it and let it rehumidify). Also, make sure the wax is left in a warm place for a few hours prio to application so the wax is malleable. In most cases only 1 layer is needed because good humidors lid fits well as it is.
Emo