I’d actually suggest tweaking the approach a bit - if you create a normal Fly PG HA setup using fly pg create, you can then modify the Postgres-ha repo to add timescale and just redeploy it (Fly PG apps are just Fly apps with extra initialisation). That way all the setup is already done for you.
There’s a walkthrough on adding timescale to an existing DB here How to Enable TimescaleDB on an Existing PostgreSQL Database | Severalnines