You don't mention if you are car camping or hiking.
If you are car camping, I like to put herbs, salt, pepper, and lemon slices inside the body cavity, then wrap in foil. In fact, if you take oil, or a knob of butter, or some bacon you can slice some potatoes, onions, and any other herbs/dense veg you like (green chile in my camp; parsnips, chives, and carrots are great too) and put them in a foil pocket. Just a tablespoon of oil/butter or one slice of bacon for 2-4 servings. The fat is carrying flavor, rather than helping the cooking. Start the fire, let it start to burn down to coals, and set the potato mixture next to the fire as you prepare the fish. Think of it as getting the cooking started. Prepare the fish, cleaning them, stuffing them, and wrapping them in foil. When the fish is ready set them directly in the coals (no need to bury them) and move the potatoes to the coals. The potato packets should be too hot to touch at this point and you will need to use a spatula, or you thought ahead and placed a couple of long sticks under the packet when you placed it next to the fire. Depending on the size of the fish, and how much contact they have with the coals 15 minutes should just about do it. Don't let them sit longer than 20. Fish them out of the coals (I use tongs for this) and open them up and let everyone serve themselves.
I like the basic fish preparation (herbs, salt, pepper, and lemon slices in the cavity) enough that when my dad gave me some fresh salmon a couple of years ago I prepared them that way, put about a tablespoon of butter in small pieces across the top of the fish, double wrapped them in foil, and ran them through a wash cycle in the dishwasher when it was time to do my lunch dishes. The dishwasher doesn't get over 200 degrees, so when the cycle stopped, the fish was fantastic.
Another one that I did this last summer with the kids came from Bon Appitite Magazine. Dress the fish, and put a herby vinaigrette in the cavity. Wrap them in bacon, and lay the fish directly on a bed of hot coals. No grill needed, no nothing, but I cheated and didn't use the vinaigrette, and I had one of those fish grilling baskets. The magazine article said to lay the bacon wrapped fish straight on the coals. Turn every 2-4 minutes until the bacon is cooked, total about 15-20 minutes, and pull fish off the coals. Eat the bacon and then tackle the fish. The kids even ate it. While I have not tried the vinaigrette, I think it would be a lot of hassle for just a bit of flavor, and the cooking fish will push moisture and fish oils out, diluting it anyway. A vinaigrette is 2/3 oil, and seems like too much trouble, and too much oil, but it would make you look cheffy when you told everyone what was for dinner. I would just stuff the cavity with parsley, green onion/chives, maybe some chopped garlic or garlic powder, a frond of dill, (the little plastic containers of fresh herbs at the local grocery store are great for this), a green chile or a sliced jalapeno, and sliced lemon/lime. If in your dictionary bacon is not a healthy fat, you can omit the bacon and cook the fish straight on the coals. But in my dictionary, bacon is an essential fat.
If you are hiking/backpacking you can make your own spice blend and just carry that. Lately I have been using a small local spice guys rubs on everything, including popcorn. Salt, sweet, spicy, I would not hesitate to put this in fresh caught, campfire cooked fish.
http://albukirkyseasonings.com/products.html
The BBQ rub and the Green Chile rub I have tried. I am loving them on chicken thighs, roasted, served hot or cold.
pat