Look for a lab sized (silicon) chip fab furnace, if one is big enough for your needs. People don't generally recognize these on eBay unless you are bidding against a science teacher.
They often are round, some are horizontal, and some vertical. Lindberg is one name. I got one with 2200F capacity, slightly over 5" diameter, by about 10-1/2" deep (vertical). It is perfect for the (wood) plane irons, and small HSS and A2 tooling I make. If I needed to do something like a knife, it would not be hard to extend the length(depth), by moulding a castable refractory lid with an extended chamber, and just always keep the tang end in the add-on section. Mine is programmable for cycles up to 8 days long, including multiple ramp up up or down of any variable time and temperature, holds- ditto, and shut down. It cost $150 a couple years ago on Ebay after watching for maybe 4 to 6 months at the time.
smt