gull busters !

GREETINGS ALL! NEW 23 SXP FAST BACK OWNER - LOVE THE BOAT - I ADDED 2 8 FOOT "BIRD SPIDERS" TO TRY TO ENCOURAGE THE SEA GULLS TO "MOVE ON DOWN THE LAKE" - MY PROBLEM - I CANNOT KEEP THEIR 6 POUND SANDBAGS BALANCED ON TOP, IN THE CENTER OF THE COVER - WHAT SOLUTIONS DO YOU USE FOR THIS...