While lots of good syrup has been made using restaurant warming pans on a concrete block arch, it will be both faster and easier to get the syrup made using a stainless steel no lead solder flat pan and you will usually make better syrup. Using warming pans there are at least 2 big issues. First, is the fact that the sides of the pan at and just above the boiling sap gets burned onto the sides, adding a slight burnt taste even to the best. The second, you have 2 ways to run more than one pan, neither is fool proof. You either find a way to move the boiling sap from pan 1 to pan 2 and then to pan 3 as it becomes more dense or you just keep adding more sap to each pan as it boils down, both methods work, both could be better.
If you are thinking a flat pan, check out the numerous pans that will be posted in the classifieds on this forum in the next several months as others expand and out grow what they have. Just either look for welded pans or check for lead in soldered pans, test kits are available at most hardware stores. Another route is check Craig's list, again be careful. A third is check to see what dealers have taken in trade. Maybe the last is to buy new, for that decide what size you think you need and search around. Look at the smaller manufacturers, some of which have adds on this forum.
Good luck, this will take your syrup making to a whole new level and a flat pan will really make syrup faster and it will be easier to get better syrup.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.