DIY heated bed for virtually free?

I wanted to add a heated bed to my printer, but in terms of priority it is not at the top of my list for now. So I created one from things I had laying around. I have two soldering irons, one of which I dont use. It is 30 watts at 120v. It was not made to be disassembled, but my grinder fixed that. After the plastic handle was opened I realized it was basically empty. All it consists of is a metal tube with a heater cartridge inside. The heater cartridge is held inside so I just used a pipe cutter to carefully trim it down to size. What I am left with is something very similar to what heats our extruders. However, it was meant to be run on 120v. I tested it with DC 20v and after several minutes it was only warm to the touch. But, this is advantageous for me because I dont have to worry about powering it via a power supply. So I used high temp wire I salvaged from a microwave and rewired it to a standard household plug. For control I am using a 10a @ 125VAC relay that only needs 5VDC to switch. I didn't feel like figuring out how to get it working with my Ramps/Mega so I am making it a stand alone system. I am using an Arduino UNO for temp control. The relay will be wired in series with the hot wire, and then connected to the Arduino. For monitoring I am using a DHT11. I also have a mechanical thermostat and several other electronic ones but this is the easiest to use in terms of mounting and compatibility. With some simple coding, most of which can be sourced from the Arduino website, I can now power the heater from the wall outlet and switch it on and off at any temperature I want. To distribute the heat, I drilled a hole into a plate of aluminum that the heater cartridge will be inserted into. The soldering iron has a removable tip so my other option is to insert some wound solid copper wire, but I need to find some and also test to see how well it will distribute.

To be honest since you can find heated beds on Ebay for like 30$ so I can't exactly recommend this. Also, if you aren't comfortable with wiring and AC circuits I would advise not to do this.
However, for me it suits my situation, and it was just a little project to take on. I am still working on the electronics. Once that is complete I will test it and report back as to how well it works, or if it is a fail.

Where your heater cartridge is mounted in aluminum plate
will be very hot , the area away from the heater will be cold,
Your heated bed will have a tremendous heat gradient that
may induce extreme warp.
What do you use atop the aluminum plate to provide a flat
printing surface?
What is under this heating plate to keep plate from heating entire printer mechanism?

Do you map your build plate to compensate for lack of flatness.

Do you really need a heated bed?