who'd a thought that there would be a dozen ways to do something so simple.

Same as Monjon, I just twist the top several times, double it and rubber band it.

Typically, I use at least 2 trash bags for liners, or a trash bag and  lightweight drybag or packliners.  I'd never trust a single layer, and my down sleeping bag is in an additional two layers.   I'd think about bringing several extra trash bags, just in case the outfitters packing for your food is using only a single liner.

Nothing at all wrong with using a backpack - I've used a large internal
frame backpack on many trips.  Your typical nylon backpack is pretty quick drying - I would normally lay the pack down so that the shoulder straps and hip-belt are "up" - that way, weven if there is some water in the bottom of your boat, the straps stay dry unless its raining.