I agree that liners are the way to go - they're not that spendy - but I think the duct tape could work. A single strip wouldn't do much, but maybe two or three strips, or a regular-width strip folded over on itself. I'm just figuring you could pretty much make a Mr. Tuffy out of duct tape. It is true that it wouldn't work as well as a Mr. Tuffy and certainly not as well as Armadilloes, but, you know, it would be wicked cheap.