The sand eel in particular can be extremely deadly on the open coast, and they do have an ability to attract bigger fish. And the perfect imitation is of course the legendary Surf Candy by Bobo Popovics. A bullet proof fly, the perfect silhouette, can be tied in any colours to match local variations (I usually just go for white belly and some shade of olive for the back) and the profile and the weight of the epoxy or UV-resin makes the fly fly like a bullet through the air.
But the sand eel in particular can also be one of the more problematic to fish, because it's the longest and thus the wing has more of a tendency to foul foul in the hook bend. I like the look of the fly when the resin or epoxy ends just over the barb of the hook. That also ensures that the imitation doesn't get too heavy, so I can cast even largish ones on 6-wt outfits (even though I often use 8-wts for this kind of fishing). And that's fine.
But bringing the epoxy/resin a ways past the hook bend just reduces the numbers of fouls infinitely (which Bob Popovics also points out), which is why I prefer as short a hook shank as possible. That means I can brings the resin past the hook bend and still tie an relatively light weight fly.
Another important aspect of preventing the wing from fouling is the cast, of course. Parallel loops and even slightly open loops can help a great deal too. Open loops can be hard to maintain on long casts as the loop morphs, but it'll still help. Plus I often like to fish these in a slight head wind, which stirs up the shallows, and sea trout like to hunt in those conditions, close to shore, +35m casts aren't really necessary.
As you can see in the PoD, I like to bring the UV resin just past the hook bend and the short shank hook keeps the head relatively short.
Hook: Ahrex SA 274 #2 in this case, but I tie them as small as an inch/inch-and-a-half on #10 and 12 hooks.
Thread. UNI mono.
Body: Mirage flash.
Wing: FishScale in colours white, wild olive and olive.
Side/eyes: CandyFoils from Pro Sportfisher, size to fit hook.
Head: Pro Sportfisher UV Resin, Thin Flex.
Have a great weekend!