The resistor that failed in the Twin Resistor experiment had the calibrated resistance wire wrapped around a hollow glass tube in the core of the resistor. The glass in the center of the resistor core melted, pooling on the downward side of the core, leaving the central upward part (roughly 2 cm in length) of the heating wire coil exposed to an air void, where it could not conduct heat well and melted.
I will look into winding my own resistor, with better heat distribution, and encasing the entire coil in alumina. I may have Kanthal wire fine enough for the job. (Essentially I have already done this with the Durapot Slab, which has cast-in thermocouples.) A small cylinder, instead of a rectangular solid, seems to be a good idea also. I haven't had much success with two-part alumina assemblies staying together at high heat. (One can see the rib part I cast as part of a Slab experiment in the upper RH of the rig set up image. It popped off at about 500 C, even though the main Slab heater was cast onto the still-wet rib portion.)