I think you might be underestimating the mini fridge chamber. If you plan on making it bigger you defitnally exceed $50. Plywood, insulation, if you plan to frame it then the cost of 2x4's, caulk, screws, hinges, and a temp controller. I prob missed a few minor things but still it all adds up.
The chest freezer is already big enough so no need to do anything but buy a temp controller around $70 maybe even less. If cost is an issue craigslist for a used chest freezer. That is what I suggest.
It is nice to build your own though.
Also to build you need tools so another thing to add to the list.
Best of luck