I put a $10 hotplate in my Weber gas grill...put a small bundle of twigs trimmed from apple trees wrapped tightly in foil on the plate..and smoke away.
The temp in the grill is roughly the ambient temp.
Each foil packet lasts about 20 minutes.
It takes a little fiddling to find the right hotplate setting. On mine it is about 50% max.
It is smokey as hell. A turkey breast will get all smoked up in about 40 minutes. Any more than that is too much smoke taste.