Another possibility for you, which I use when I am doing long boils (I only have a 7.5 gal Turkey Fryer) is to use two pots.
If I have too much wort for my kettle, I will pour the remaining wort (1-1.5 gal) into a big stockpot, and boil it on the stove. When I get down to about 10-15 minutes, I add the wort on the stove to the turkey fryer. I don't think you need to do that, and you could boil two separate batches, and add them to the same carboy. That might get you up to 5ish gallons without adding any more junk (assuming you already have a stockpot).