Go to Wal-mart and get the real pool noodles. Those dollar store versions are no good.
Seriously, I have a nice blue one I've been using for 4 years. I do not cover the whole bottom of the slide. I put an 8" piece of noodle on the corner right above the storage compartment.
This weekend I saw a camper with 2 noodles on a slide out. They started about halfway down the slide and he had them notched with 45 degree angles so they could turn the corner at the bottom. One on each end of the slide and the almost touched in the middle.