Old camellias have old habits that die hard, but if you keep pruning the unwanted shoots, your camellias should get the message, and realize they are supposed to be tree-like now. I don't think plastic wrap is necessary, and it doesn't sound very attractive. All you need is a bit of patience as you re-train your shrubs.

Camellias are slow-growing plants that respond well to being pruned to a desired shape, but it doesn't happen overnight.

Good luck,

Virginia

I also just cut off the unwanted bottom leaves when they leaf'ed out in my tree form camellia. Happened sporadically with a japonica called Debutante I think.