The candle heater is interesting. I'd mount it away from the wall about an inch or so. That way all sides can radiate heat. For more heat extraction the flue could be a long pipe, possibly folded, but it needs to be able to have the soot cleaned out. My biggest question is what happens when the box gets hot enough to melt wax?

Where's the containment in case of meltdown? I'd hate to wake up in the morning half way through a wax job.
A wide wick oil lamp could be used in place of the candle, a #3 Queen Anne lantern burner has a 1.5” wick width. Unfortunately lanterns use a spillable fuel. Instead of the box construction, I'd use an auto exhaust pipe the size of the base of the oil lamp's globe. For a #3 that is 3”. Of course that auto exhaust pipe would need a through roof fitting befitting a boat fireplace for where it exits the TD. I'd also treat it as a fire hazard and have heat reflective and fireproof wall and ceiling coverings near it.
Of course if you use this idea you are on your own, it's just an unfinished concept and not a certified safe design. It's your fault if you burn your trailer down or asphyxiate yourself. Using a CO/CO2/Smoke detector would be a must.
BTW, I'm a fan of the Espar type heaters, except for the cost. They are available in both diesel and gas fired variants and sip fuel. However they do need yearly cleaning maintenance.