Buy (2) 2"x10" boards 8' long, determine how high you want it and cut that amount less 1 3/4" off of each 2"x10" for end legs.
(15" high would equal a cut of 13 1/4")
I suggest cutting a half circle with jig saw on one end of each leg piece so there are four contacts on the ground to increase stability. Or you may cut a V instead. V is probably easer as can be done with straight saw. Maintain at least one inch from each edge of V to each edge of 2"x10" legs pieces for contact to ground.
Take one of the long pieces and attach the legs to each end approximately 6" from the edge. I would predrill and use coated deck screws for attaching as they don't bleed and stain the wood and will provide a strong joint Try to get 4" deck screws if you can, 3 1/2" would probably work.
Take the remaining long piece and cut to fit between the legs. Attach to the bottom of the bench in a T fashion. Drill and screw from the top of the bench and from the legs on each end.
The lumber will probably be wet and heavy, you may want to let it dry a day or two before you put it together. You would be surprised how much lighter it gets. Thus easier to handle.
Project can be stained, and waterproofed or painted.
Dont worry about knots - they add character as long as they aren't big holes where attach points are. Could weaken joints at attach points.
Talk to the lumberman and paint department for additional help.