Just seal the foil well and set it straight on the coals.  10 minutes a side usually works for me, with about 2-3 lbs of fish per package.  Use a couple of forked sticks to turn/remove the foil packs.  Just be sure that you have a big bed of coals before you start.  It's OK if they are starting to die down by the time you flip the foil packs.

Baked Trout:

Heavy Duty Aluminum Foil
Squeeze margarine
Salt & Pepper
Dill
Fresh Lemon (about 1/2 lemon per foil pack)
Fresh Onion (about 1/2 onion per foil pack)
Bacon (4 slices per pack)

Lay out your foil on a flat spot in camp.  Squeeze a little margarine right where you are going to set the fish (down towards the end).  Lay 2 strips bacon in this same spot, then slice onion, seperate into rings and put about 1/4 onion in this same area.  Slice your lemon very thin and lay 1/4 of the slices here, too.  Anytime during this process is a good time to accidentally kick a little forest duff into the foil.  Now cut the trout fillets as needed to fit into the foil.  Season both sides to taste with salt, pepper, and dill and lay the fillets on the butter/bacon/onion/lemon.  Then place another 1/4 of the lemon slices on top of the fish, followed by 1/4 of the onion and 2 more slices of bacon.  Drizzle the top of all this with more squeeze margarine.  Carefully begin to wrap the foil around this concoction.  You want 3 layers of foil around it so start with a pretty long sheet.  Then carefully roll up the sides of the foil and seal it well by crimping it down hard.  Bake directly on coals for 10 min per side.

You can sub lemon juice for the fresh lemon and even sub dehydrated onion for the fresh onion if bringing fresh is a problem.  If you only have room to bring one of them fresh, bring the fresh onion and sub lemon juice for the lemon.