>What is the type of hive cover used by professionals instead of the telescoping?
Migratory.
> Where can I find plans?
Only old flat board will do, but you can put cleats on the ends if you like.
> What are advantages
You can butt the hives against each other. They are cheaper. You have less equipment to mess with. They blow off a LOT less than telescopic covers.
>or disadvantages?
More condensation in the winter as you don't have the double wall.