... I tend to disagree.
I have a stone-age oven with no convection at all.
I still make biltong in convection-Mode: I have the oven at 50°C and just put in a externally powered fan. Works like a charm. I added 8 Sensors once to see if the heat is even and nice and it is. MUCH better than the simple non-Convecting.
Yes, my PC-Fan would die at 100°C or so, but thats only because they are plastic... Take an AC-Motor (Rated to the temperature of the oven) with fan and a cable rated for your temperature and i dont see why one shouldnt be able to add this retrofitted...
DIY for sure and a hassle but possible i would say...
Easy with low temp, hard with high temp...
I mean, if you want only the convection one could do it with a magnetic coupled fanblade thru the window:
Have a motor with a bar spinning in front of the window of the oven.
2 strong magnets on this bar.
All is cool there and so the neodym magnets dont suffer.
Now, have a iron par mounted somehow on the inside of the oven cery close to the window and able to spin freely (Ball bearing with no plastics. Ceramics as an example).
If the outside-motor spins, it will drag along the iron bar on the inside. Add a fanblade to this bar and you have your convection inside. Easy peasy...

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Tinkerer by heart, archer by choice and electronics engineer by trade.