Attach a 2x6 flat across the rafters and use oversized eye hooks and washers to secure into the flat board.
If the porch swing will be perpendicular to the rafters then attach a 2x6 perpendicularly on the rafters which is 2-4 feet longer than the porch swing. Attach it using 2 screws per joist.
Now attach eye hook bolts to the flat 2x6 using oversized washers and double the nut to prevent unthreading.
If your porch swing is planned to run parallel with the rafters then attach 2 shorter 2x6 boards perpendicular to the the rafters and attach eye hook bolts.
Paint it white and it will be practically invisible once the shiplap ceiling goes around the board(s).
If you want it to be more invisible from the get-go then you could attach a 2x6 between the rafters and secure it using metal brackets. My concern would be whether two 2x6 rafters will be able to sufficiently withstand a fully loaded porch swing.