I've always had a beaver tail (my dad calls it a turtle board). It hangs with straps from the frame and D rings. I like keeping hard items, like groovers or propane tanks, off the floor. I'm almost always called on to bring my kitchen, which is an army surplus dry box, that fits snugly in the rear of the boat (14' SOTAR). I can pile other stuff on the top of that.
As far as securing it all - I used to have a cargo net but got rid of it after my fingers got caught it in while I was swimming Lava. Nothing broken but shredded some of my fingers. My brother-in-law just uses a bunch of straps. I think the King Sling, a tarp, or something similar, is the best way to go.
When the river calls, the wise one listen.