Tennis balls are probably the best solution. Before I learned of using them, I used blue painters' tape wound around the gutters, extended beyond the ends. Stayed secure all winter yet did not leave any glue when removed in the Spring.
2015 Shamrock 231KSS, Oyster shell
2016 Chev Silverado LT Z71 Ext. Cab, 4x4, 5.3L V/8
Reese Straight Line WDH, 2000W Honda Gen.
2 Old Town kayaks & 2 Boston Terriers - Zoey/Gonzo
Lar & Ruth
55 nights camping in 2015, 45 in 2016