Shorting the new Bitcoin ETFs will cost you some money. Borrow rates currently around 50%, if you can get availability. BTCO IBIT, HODL, BRRR all about the same rate. Guessing there is limited availability as these just became available, so hopefully it will drop soon.
If you want to avoid these fees for BTC and ETH, you can short the future or micro future for these on the CME.
The borrow/lend rates would be baked into every cash or derivative. Otherwise, there would a fairly riskless arbs - not retail, but for the arb folks.
BITI is futures not spot....and terrible spread for options. However I think I asked this...can one long BITO shares and long BITI shares and not be considered a box trade? (Edited)
Not a Box trade and BITI is an ETF listed on ARCA- Short Bitcoin Strategy ETF - https://www.proshares.com/our-etfs/leveraged-and-inverse/biti