Are those controls all self contained?? Other than the relays to control the heating elements?Nope, I never built one myself. Waaay back when I first got involved in knifemaking the wife decided I needed to be making her kitchen knives of SS, so I needed an oven. I placed an order for the EvenHeat oven. It's worked just fine.
"IF" you want a PID controller that seems to be the best around: https://www.kilncontrol.com/shop/conversion-kits/
OR, a step down from that but has been around for a LONG time:
Or with fewer features at less cost (this is the one I have):
Here is a link that gives a brief description of each controller.