I was thinking about doing this as well at one point. I was planning on getting one of the 6" deep strap pans we used to use in a commercial kitchen (Probably 2'x3'x6") and build a pit out of bricks, then fire it with wood. I get wood for free so then I wouldn't be paying for all the propane, and it would need to boil for a long time, but the pan would give me more surface area. I'd just keep pouring in more sap as it boiled down.
Unfortunately I only have 1 maple tree and though I could get enough sap out of it over the whole season, sap does not store well, so I couldn't save it all up and do it in one day easily.