Is there current in this lake, specifically by your dock?
If so, that current and 20-30 of rope will push that tree all over the place. Where it finally ends up nobody knows. If you want it to stay put your going to need a ton of weight.
I think your better off building up that cover over time. Every year stick your Christmas tree in a 5 gallon bucket and fill the bucket with concrete, remove bucket, and drop it off the dock. Get all your neighbors trees too.
Or you could stick a 30' tall tree ina 55 gallon barrel and fill the barrel with concrete, push that off the dock and pray it stands up.
HOI Crappie Club
Where family and friends come to compete for a little more than bragging rights.
Quick, someone teach me how to fish so I can win this tournament!!!