I would recommend against the use of LDPE. They do sell it as grafting tape, but it's not even biodegradeable, and it really works against the success of the graft if the intended use is to minimize transpiration.
When you graft, you can wrap the scion in parafilm or use a zip-top bag and put some moisture in it. In either case, the goal is to reduce transpiration, not to prevent it. By preventing it, you can smother the scion, keeping the transpirational flow from occurring and preventing the action of moisture supply from the rootstock occurring. You can think of it as capillary action, when you put a paper towel in water, it wicks water up. If the towel is already wet, the effect is reduced. The idea is to allow flow to occur, but to prevent the scion from drying out prematurely.
Parafilm or other paraffin waxes are used frequently for this purpose, as they limit the transpiration, and are biodegradeable.