I had considered your method when I built my first HT oven. The reason I opted not to run the elements through the wall is that I could not come up with what I felt was a reasonable design for doing so.
Feeding the elements through an insulating medium seemed likely to result in overheating and rapid burnout, due to the elements' inability to shed heat by radiation from within the hole through the IFB.
I'd thought about twisting the elements, either to connect them in the oven, or to feed them out with increased cross-sectional area to reduce the heat generation where they pass through the IFB, but couldn't really see a way round a sudden reduction in effective cross-section at the hot end of the twist.
In the end, I went for the studding.
So far, I am not aware of a failure attributable to the studding method. Only one of my ovens has suffered an element burnout, and this was not at the studding, but on the straight section of the tail.
The stainless studding is M6, so has many times the cross-sectional area of the 1.29mm (16 AWG) Kanthal A1. If we assume the core diameter of the M6 x 1.0 studding to be 4mm (od- 2xthread pitch), it still has over 10 times the area of the Kanthal. Heat generation within the IFB will therefore be minimal.
If it becomes a problem, the issue with the stainless washers and studding seems more likely to be a loss of contact pressure than excessive force on the Kanthal; the expansion coefficient of the stainless is significantly higher than that of the Kanthal.
My concern has always been progressive loss of oxide layer causing poor enough contact to affect performance and/or cause burnout. It's something that I'd expect to see to some degree, but I don't know whether it will happen fast enough to cause a problem in the real world. I don't want to go chasing after a "better" way if the current way is good enough.
I considered the possibility of deforming the Kanthal where it is clamped between the washers. I use the large diameter washers in part to provide a fairly progressive reduction in clamping pressure as the distance from the studding increases.
My ovens run a contactor in series with the SSR; I have built a number of HT ovens, all of which have gone to people I like and would rather keep alive. I do not trust an SSR to provide the isolation necessary to ensure their survival, so fit the contactor in a No-Volt-Release circuit with the door switch in the hold-in circuit. Pressing the stop button or opening the door drops out the contactor until the start button is pressed whilst the door is closed. The contactor provides a real air gap, which the SSR alone cannot.
I should point out that I'm in the UK, so we have one "hot" leg at 230V and one Neutral leg at at Ground potential. I only fit the SSR in the hot leg, but the contactor breaks both the live and neutral (I gather you have 2 hot legs, so dual SSRs make more sense over there).
I also fit mains plugs with integral Ground Fault Circuit Breakers. I have racks. Even so, I still feel it is reasonable to take any simple additional measures to reduce the likelihood of shorting the elements that I can come up with, especially when they cost virtually nothing and do not detract from the performance in any way thay I can see; the counterbore is only to the same depth as the element grooves and the flat bit to cut the counterbores cost 1.43 GBP (about $2.30, which works out under 50 cents an oven).
It's not just safety; if an element gets shorted and fails, it'll probably be me that gets to change it and it's not really a job I relish.
At the moment, 4 of my ovens are with guys that will give them some use and who should be able to break them much quicker than I could. All are makers, deriving at least some of their income from knifemaking. I've had some useful feedback from them and have been able to make some improvements. Once I'm reasonably happy the design works well enough and reliably enough, I'll stick a write-up on the web and move on to the next thing that interests me.
