Here is a nasty idea: have your downvote power be the difference of your upvote power.
Aka, if your bar shows 70%, then your upvote is 70% and your downvote is 30%. Meaning, if you want to downvote, you need to spend power in upvotes.
Downvoting will deplete the bar as normal, meaning it gets stronger the more you use it, as opposed to upvoting.
But either way, it makes little difference if you don't have at least 1000 SP. I personally use the abuse form Steemcleaners is using.