Really?? If you really think paper towels are the most dangerous substrate then I would recommend using no substrate at all! God forbid they just start eating it...
Aspen ingestion is no big deal. A few pieces here and there aren't going to hurt it.
Majority of keepers here use some sort of paper substrate. Thinking it is the most dangerous substrate is kind of ridiculous. Sure accidents do happen, but using newspaper vs. paper towel isn't going to change that. Just watch your snakes while they are eating and make sure they aren't eating the substrate.
I use paper towels in all my enclosures. Have been for the past 4 years with not one single issue. Not saying it won't happen, but I don't expect all my snakes to start eating the paper towel one day.
Sorry I just think this is not the best piece of advice. Paper towel is not a death sentence.