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Here is a question: Did you check that your therm is accurate? I would think with an oven element you should have been able to get a lot hotter than 170°.
Is the oven element 110 volt or 220 volt? If its 220 and running on110 you won't get enough heat out of it. Don't get discouraged though, it will work out in the end. Just need the bugs worked out.
The shear mass of the fridge is going to be tough to overcome when it's cold outside. You may want to wheel the fridge into the garage to warm it up, before you smoke. Bring the temp up from 60° to 220° is much easier than bringing the temp up from 22° to 220°.
If you're going to use this fridge outside, you may consider lining the bottom with bricks for a heat sink, and using a larger heating element. Once you get this to temp, you should be able to maintain it.
Post a pic of your build. This may help someone figure it out for you.