I used my biggest drill bit to cut through the aluminum, then used a grinding attachment on my drill to grind it our to the appropriate size. This was much cheaper than buying a larger drill bit.
The handles are not covered on my version of this, I only ran the insulation and aluminum to just beneath the handles.
Lastly, at least for me, this was not designed to save gas necessarily but rather to get the mash tun to hold its heat better so that mashing requires less fuss... meaning I have to add heat less frequently making mashing the mash process simpilier. I still need to heat it, but usually only 1 time in the summer, and 2 times in the winter.