You really need a thermowell....
I have this "automated" setup using an Arduino reporting to COSM I made and documented in the DIY forum, so i can keep track of my ferm temps more consistently. Check out the thread if interested, total cost is about $60-70, not counting tools like soldering iron/wire.
Its not that interesting now since both probes are just dangling in the air since I have nothing fermenting at this moment in my monitored bucket, i have another bucket going but i havent modded the lid to take the thermowell yet.
If you set them to 30 days you can see on Jan 3rd when i started my lager, and when i brought it in for a D-rest, then where it gets spikey again is where i took out the sensor and kegged to lager...never got quite as warm as i wanted but it'll have to do.
You can see even with my garage having 3-4 degree swings, it really didnt effect the beer temperature all that much.