Etching tank
DigiTan, I think your best bet is to use a glass-enclosed aquarium heater.
A quick google for, "aquarium heater," yielded the following from a major retailer (that I do not, however, recommend doing business with):
"A direct-reading, thermometer-type scale makes presetting temperatures easy
Completely waterproof and totally submersible
Double-sealed and needs no rubber or plastic cap over the adjustment stem
Compact design
Available in 6-, 10-, 12- and 14-inch lengths
Choose from the following wattages: 50, 100, 150, 200, 300 watts."
You will have to check for absence of exposed metal and adequate sealing for FeCl, but these are made for permanent immersion in WATER, so are at least well sealed. I would think a visit to your local aquarium supply store would allow you to inspect the type of seal. 300 watts is certainly way more than you need for an etching tank.
I made an etching tank many, many years ago out of bare epoxy board and epoxy adhesive and it worked fine. I chose to make it thin to minimize etching fluid volume, vertical to promote fluid circulation and agitation using an aquarium air pump and porous air distributor to speed up the process, and applied a surplus, flat, flexible, red rubber area heater to the flat side of the tank. It worked fine except that I could not see through the tank wall to watch etching progress. You have to leave lots of headroom and shield the top to avoid spatter that can stain and damage almost anything in the area.
If I was to do it today, I would fabricate a tank out of 3/16" or 1/4" clear plastic sheet, using adhesive specifically made for the material, and use a flat rubber heater element operating directly on 115 VAC with a thermostat mounted on the outside of the tank in series with the element. I don't think 120 degrees F would be a problem, but I would check with the plastic supplier. I believe that acrylic would be very satisfactory and not very expensive. With this approach you will have to settle for slow, gentle heating because plastic is a poor heat conductor. If you are in a hurry, you could preheat the etching fluid in a glass vessel and just use the tank heater to maintain temperature during etching.
If you use a porous air distribution element (which I highly recommend), be sure to test a sample of the type you want to use by soaking it in etching fluid (it is not "etchant") because some will soften and break up in FeCl.
I don't think you can use any type of metal in contact with the FeCl, which is the reason I recommend glass-enclosed aquarium heaters.
I think you could use the same basic layout with extra space at each end to make room for vertically oriented aquarium heaters, keeping the feed-through seals and wire completely out of the etching fluid.
Actually, at work, we just used a pyrex baking pan sitting on a flat glass secretary's leg warmer panel for years. Worked fine, but messy, bulky, and inefficient
Have fun.
awright.