I just went out and measured: mine is 10.5" wide and 12" deep.
So, at least one nice sized Plano would fit inside but I don't know the 3740's dimensions.
The Tackleweb I have fits "inside" the slightly wider perimeter size of the back of my seat so it isn't sticking out, doesn't impede me swiveling around to access either it or something in the tankwell.
And, it is not as flat as it appears to be in the photo; that is, it blouses out so something other than a flat object (like a book or a magazine shape) could be placed in it. Not that you'd want to . . . but as an example I guess what I am saying is you could likely cram a football in there.
It has a Velcro strip across the top to secure it.
This company is one of the better "finds" for me in the after-market accessories for kayaks. Well built and relatively cheap compared to anything Native or Hobie has.
Another GREAT source for after-market kayak stuff is The Kayak Fishing Store, link below. If you are trying to adhere to a financial budget, DON'T click below. Ha!
Brad Great after-market source