I only use a small piece of cedar on the grill and put the salmon on aluminum foil. Saves a lot of cedar. You don't need that much smoke to impart the flavor and you certainly don't need a piece that is the size of your fillet.

I just cut about a 2" piece off of a piece of 1 x 8 and soak it for an hour. Put it on the hot grill to get it smoking about 5-6 minutes before you put the seasoned fish on the grill on the aluminum foil tray. Keep the heat going and it's done and pleasantly smoked in 10-14 minutes. You can adjust the size of the piece of cedar for the amount of time you expect to cook your fillet, but that small piece will smoke for about 15 minutes.

I leave the partially burned cedar in the grill after I turn it off and it smokes a little more when we barbeque steaks or chicken. It's goooood.

I will try the alder next time. I don't see why it wouldn't be good too.