Hedging these highly-valued but illiquid stocks is difficult and the only sure fire way is to build the short into the original fundraising deal. As the saying in SV goes "Your valuation, my terms".
The long and short of it is that opportunities to hedge the risk of a tumble in unicorn valuations are rare, which is one reason why the sticker prices are only going higher. Trying to sell stock in the nascent secondary market is the only sure-fire way to de-risk, and that usually requires company permission. One thing to remember is that there is more to the valuation story than the headline numbers. As venture capitalists are fond of saying to company founders: “Your valuation, my terms.” By stretching anti-dilution clauses and other deal provisions, investors in many of these latest fundraising rounds appear to be demanding additional downside protections that guarantee them extra shares if the valuation falls in the future. This, then, is the best way to short a unicorn: build the short into the original fundraising deal.