Some design alternatives: • Use lime cement rather than portland cement for the mortar. Portland will be damaged over time by the 120° temperature and may spall explosively, causing injury, particularly if you embed nichrome heating elements in it. • Similarly, don't use plaster of Paris inside the oven. It will also degrade over time at that temperature. Waterglass would work to fix tiles to wood but will foam up wh…
It occurred to me that this was a potential use case for obsolete CPUs, which are often conveniently removable from ZIF sockets in obsolete computers. Almost all chips have "clamping diodes" on every pin except their power rail pins, in order to keep those pins within 600mV of the power rails to prevent damage. If you connect the power rails backwards, the clamping diodes are forward biased and start to conduct; at least with old SSI chips and EPROMs, this can get the chip quite hot. Normally the clamping diodes are only rated for 0.5mA or so each but I'm guessing that often they won't actually melt until quite a bit more than that. In normal use, current microprocessors commonly draw over 100 amps at 1.2 volts or so, for which purpose they are packaged in ceramic with metal heatsink surfaces. I doubt you can get the whole 100+ amps through their protection diodes, but maybe you could get enough heat to cook with. Certainly the chips will withstand the 120° temperatures of this oven with no difficulty.
I should also mention that lime cement needs access to carbon dioxide from air to cure. If you sandwich it between ceramic tiles, it won't cure. Similarly for sealing it with waterglass: let it cure for a month first.
There are other candidate cements that are also safe for oven use, including some which can be made from common materials. Many that are too expensive for buildings because they cost over a thousand dollars per tonne would be fine for this purpose. Wood ash makes a perfectly adequate substitute for lime cement, for example. Some alternatives can cure without air; I found a very interesting recipe for "fire mud" in an Ayurveda textbook which is largely iron oxide and buffalo milk held together until heating with a little gum arabic, the reaction product presumably being a calcium iron phosphate similar to anapaite and some rodent teeth. Some industrial castable refractories use a similar formula, though aluminum phosphate is a more common backbone.
(Why would an Ayurveda textbook have a recipe for castable refractories, and why should we believe that it works? I haven't tried it, but its claimed use is for connecting ceramic vessels together to make distillation and calcination apparatus, and you'd hope that practitioners would notice if their distillation apparatus were unusable—even if they can't tell if the drugs they distill with it kill patients.)