I want to create optimised portfolios in five different countries and calculate if the inclusion of cryptocurrencies or other alternative assets contribute to the diversification. I want to use something as a proxy for the risk free-rate that will be comparable to all the economies to calculate metrics such as the sharpe or the sortino ratios. I thought about the T-Bills but each of these countries (US, UK, AUS, GER, CHI) have T-bills of different durations and for some of them I cannot even find data. Can someone share some advice? I use weekly frequency in my returns if that is helpful.