There must have been people before me who had the same question, i.e.: suppose I want to 1:1 short a fixed number of BTC, which (and where) is the cheapest way to short?
What you meant is that you do not want to leverage but go short so if the price of bitcoin is falling, you will be gaining. The best future trading you can do is to never leverage, just go 1:1 which means you do not borrow, only what will be deducted from you is the transaction fee used and nothing more as you did not borrow any money to leverage.
But, it will be good to think of this very well, opening position in short direction might not be good for long term, it can be better if used for only short term. Not an advice. DYOR.