Yeah a dremel is the best tool for this. I had a few like this on my D2. Cut the heads off the nuts to remove the tank, and then cut the nutserts off the body from underneath. I've got plenty of spares plus an improvised tool to set them so PM me if you need a few, or cable tie it for now and swing by next time you're passing
Alternatively you could also melt through the plastic with a soldering iron, if you don't want to put a saw near the bottom one. Or heat up an old kitchen knife with a blowtorch and melt it with that.