towel bar too short

RE: Door Handle With Towel Bar Insert, 2-1/4"
Product Code: 10-133

got a set....they fit the shower door holes, the towel bar slides in fine....but now the bar seems a tad short....

any suggestions what I could stuff in the door handle openings to reduce the sliding back and forth...it would have to be something that would obviously survive getting wet..

Know this may be a long shot but thought I'd ask in case other folks had the same problem in past..
any help appreciated...

thanks