I've done various types of ramps here...ones with rungs and ones without rungs. What I like best, and what is simplest (in my opinion) is just tacking scrap shingle material along a wide plank/board (overlapped a bit, just like shingles on a roof). It's very easy for the birds to get grip/traction and it's easy to hose/clean off. For chickens I've just used a 6" deckboard (shingle material sticking out on sides about an inch, so more like 8" total). I'm not a duck person, so don't know for sure - but would think maybe a wider ramp would be better for ducks??? So if you don't have wider planks, maybe two boards side-by-side???
Oh...and I just use a single screw through the plank, angled, into the side of the coop to hold in place. If you want better stability, nail or screw a small resting strip of board in place to rest the ramp on, and then screw the ramp on.