Everyone has a trick for this, but I’ve found that there is nothing better than an inflatable exercise ball to support your boat tarp or cover.
The balls come in different sizes, and you can often find them for less than $20.
It’s easy to blow them up to the desired size and shove them wherever they’re needed: between boat seats, between seats and consoles, or on the front casting deck.
The rubber surface grips well and won’t leave marks on your boat.
I haven’t found anything else that’s gentler on the boat cover or easier to position and install.