You should try a 1 strap Booster Strap.
The OP is talking about an Elastic after market Booster Strap not a stock static power strap.
I would also add if you cannot ski with the top of your boots reasonably snug, I don't mean really tight, something is amiss! Boot too stiff? ramp angle wrong, Too much forward lean,
Shape of rear of boot does not let your heel seat in heel pocket correctly, or could be all of the above!
I actually have two very different pairs of boots, but prefer both loose for everyday skiing.
I can ski very well with my boots cranked down tight. I just prefer them loose when casually cruising. if I encounter something or I crank them down. Then, I loosen them back up when the serious event has been conquered. Any kind of strap is simply something else to mettle with while cranking them down or loosening them back up that I'd rather not have to mettle with. I don't "need" my boots tight to ski anything at all, any terrain any course. I do ski steep, icy stuff better with them tight. Bumps, I prefer them not quite as tight as racing though.