Step 15: Performance Improvements
I then raised the rack about 1.5" closer to the top heating element, and sealed most of the holes and tiny seams inside the oven using aluminium tape. This isn't the smartest idea since I had no idea what temperature the adhesive can withstand, but it seems to be OK.
After these changes, the temperature reached 225 degrees Celcius in 360 seconds, and 260 degrees Celcius in 570 seconds.
Then I applied a layer of aluminium tape to the front glass door, covering the bottom 2/3 of the glass door. I then performed another test: 225 degrees Celcius in 320 seconds, and 275 in 560 seconds.
This is just fast enough for reflow soldering according to the temperature timeline that I am following, but more improvements would need to be made.
I stuffed the inside of the oven with a layer of pink fiberglass insulation. This stuff is not electrically conductive and can handle a temperature of 500 degrees Celcius. It is safe for use in my situation.
I also found a large brick to place in the oven. The purpose of the brick is to occupy volume so there is less air inside the oven to heat. The brick is covered with aluminium foil to stop it from absorbing heat too fast. This brick actually did not get very hot, it is actually cooler than the surface of the glass door. It does its job exactly as I expected it to.
I am very happy with the final results: 225 degrees Celcius in 300 seconds, and 275 degrees in 540 seconds. I can reach the end of the "soak" stage in just 3.5 minutes.