David's right on. Evaporators do best when they have maximum surface area between the sap and the fire, minimal surface area on the sides to cold air, hottest fire you can make and direct contact from the flames on the pan bottom (no stove steel in between).
Given that you're likely not going to rebuild your rig mid-season, I'd run the sap really low in that pan. Maybe 1 to 1.5" deep. That
should boil decently. You'll have to add sap way more often but it should boil faster overall. If a steam table pan (hotel buffet pan) would fit on your stove that would be a cheap way to upgrade. Split you
wood to wrist sized and aim for the firebox to be about half wood and half air space. Criss cross the wood for lots of air flow. I
feed wood into my rig about every 7 minutes but I'm using mainly softwood. Hardwood may increase that to 10-15 min but I'm not sure. You want a fire that you'd never want in your
wood stove in your basement. Having a grate to let the ashes drop down and keep air able to go under the fire is also very helpful.
Good luck and enjoy the sweetness!