It has been over 18 years since I cooked Trout on the grill fresh from the lake. If I remember correctly I had a medium (350º) hot fire. Layed the Trout on a piece of aluminum foil, placed two medium slabs of butter and placed a piece of Bacon inside the fish. Closed the Trout up and sealed the aluminum foil.
Cooked on the grill turning once for about 15-20 minutes. That is where it gets blurry, I cannot remember for sure how long I cooked them so take that with a grain of salt and some testing.
These were Rainbows, maybe 10" long, not very big.