When I moved last year and got rid of my workshop tools I apparently also got rid of my soldering gun and iron. In any case I couldn't find them. I suspect that they would be much too hot for PET plastic - there seems to be a narrow temperature band when the stuff welds and doesn't char. Don't even the cheap soldering irons have a temperature control of some kind built in? If so, how would dropping the voltage affect the temperature?Another way to weld the plastic bottle is to use a cheap soldering iron, the lower the wattage the better. Then you can use the extra bottle material cut in strips for welding rod to fill in the gaps.
I have used a variac to control the heat of the soldering iron. you may be able to use a dimmer to turn it down a bit also.
The DIY drop checker was still working this morning, with no leaks that I can detect. It finally worked loose from the glass, probably due to the GDA on the glass, and began floating around the tank. So, I removed it at noon - I consider it to be a success!