You can use a rocket stove base to boil your sap - you just need a good pan to distribute the heat from the mighty mighty rocket flame. This will let you evaporate much more sap with much less wood.
The current Rocket Oven Kickstarter https://www.kickstarter.com/projects/paulwheaton/rocket-ovens-feature-length-documentary?ref=bq4tzb
has surpassed the $20,000 stretch goal, so backers at ANY level (even $1) will get access to a microdocumentary about making a cooking grate to go on top of a rocket oven base. For the whole thing it's just $15 (for streaming access).
At $15, you get access to gobs of cool video, actually - check it out because stuff is added every couple of days. (Kickstarter ends on July 26)