Try a dry brine. 4:1 brown sugar to kosher salt. Cover bottom of non-reactive dish with layer of brine. Place fillet on Brine. Cover fillet with brine. Add another layer of fish cover repeat as needed. Place in fridge for 6-8 hours.
Take out rinse well pat dry, season with whatever you like. Use powdered spices. Garlic, onion, black pepper, etc.
Place on a drying rack and allow the pellicle to form. This can take 2-4 hours.
Smoke. I’d use a mild wood. Apple, cherry, pecan, alder, peach are all good choices.
I like to smoke fish at lower temps and I use a step method. I start the smoker at 120-130 for the first hour. Then I upnthe pit temp 10 degrees every hour until I reach the desired IT (usually 145) for the fish or until the pit temp hits 170-180. You do not want your pit temp any higher as you’ll start rendering fat out.