First, let it be known, that I'm a software guy who dabbles in hardware.  I'm not an EE and in fact, I have never even taken a high school physics course.  You should suspect anything I say and verify with someone who does have better knowledge of hardware and electricity.

Some of the things I have done with this build involve disabling normal safety checks.  You should be very wary of these things and take approprate measures, including but not limited to the following, if you choose to implement them.

We are talking about using mains voltage (120v or 240v depending on your location), computer control and a lot of heat.  Precautions must be taken, especially if you plan on running an oven unattended.  Please, do not construct an oven if you are not comfortable with these issues.

Any home made device like this should incorporate hardware interlocks or fail-safes.  In this case, a basic fail safe can be made for under $20.  With two pieces of hardware and some wire.
Buy a contactor (20-30amps or larger depending on the amperage draw of your oven) with a line voltage coil (120v or 220v) and a thermal fuse.  The thermal fuse does not need to be high amperage, 1a-5a would be perfectly fine, as the fuse will only be inline with the coil on the contactor.  Make sure the voltage rating on the thermal fuse is appropriate to your situation.

Commercial grade contactors with 120v coils, as of the time of this writing, are available on ebay for under $15 shipped.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=%22120v+coil%22+contactor+%2820a%2C+25a%2C+30a%29&_sacat=0&LH_TitleDesc=0&_sop=15&_osacat=0&_odkw=%22120v+coil%22+contactor+%2820a%2C+30a%29

Attach hot to the contactor power lugs.  Also attach a thermally insulated wire to hot, attach the thermal fuse (do not solder, crimps only) to the wire, and locate the thermal fuse somewhere on the oven or in the oven case.  The other side of the thermal fuse should go back to the coil on the contactor, and the other side of the coil goes to neutral. 

All this does is turn on the coil when power is applied.  If the thermal fuse breaks, power to the coil is removed and the contactor opens, removing power to the oven and all electronics.  It's very simple and can help reduce the chance of, though not completely eliminate, damage and destruction if things go wrong.

I am specifically not recommending a location for the thermal fuse or a rating in Celcius.  How you construct your oven and what you use if for will influence proper thermal fuse sizing and location.

I have not done this yet, but it's on my short list of things to do.

It should also go without saying, but the oven should be well grounded and a plug with a ground pin should be used.  Don't cut corners on safety.