Old cotton bedsheets have worked well for me for the past few years.
Cut into 10 by 16 sheets (with no holes or weak spots), folded and the top and side sealed with duct tape or red tyvek/vapour barrier sealing tape, they give me a bag 8" wide by 10" deep. They slip nicely over a top cluster cut down to a half dozen buds, and I twist-tie the bottom. After a week and a half to two weeks, I usually have to remove the bags and re-trim the heads, because new buds form rapidly and can overfill the covers.
The resultant seed has come true to the parent plant, so it appears they're effective against both pollinating bugs and drifting pollen.
They do get heavy and sag in rain, sometimes keeping a permanent tilt at the top of the plant, but that hasn't hurt the seed or the tobacco leaf.
If I'm late putting the bags on and flowers have actually emerged, or if flowers are present when re-trimming the plants, I wash up before going on to another variety, to reduce the risk of me cross-pollinating some plants.