Global X just launched Bitcoin Trend Strategy ETF (BTRN), which is based on CoinDesk Bitcoin Indicator index. Here is their their ETF methodology: https://www.globalxetfs.com/funds/btrn/

The composition of the Index is adjusted weekly, each Wednesday (prior calculation day if Wednesday is a market holiday).

The index methodology is attached here. In summary, they publish BTI and ETI for Bitcoin and Ethereum indicator respectively. Both are assigned a value in 5 categories: 1, 0.5, 0, -0.5 and -1.