Cheap and easy is the "drill a small hole" method.
Figure out where the attachment points need to be.
You can use a temporary pair of nails to hold up an unoccupied hammock.
Drill the holes, drill into the upper half of the joist. Then string thru a contionous loop of 1/8" amsteel, and larkshead it to the joist. Clip your hammock to the continous loop.
No lumber to buy, or attach to the home. No need for multiple boards. Or worrying about how well your attachment job will hold.
The flooring above is obviously holding the weight and sharing the stress, or the joists/rafters would have folded over by now.
I believe Knotty did the drill technique in his basement?
Sometimes simpler is better.