I chose Oregon Pine (Douglas fir) for the basic structure. It is traditionally used in boat construction for centuries. This wood is hard and straight grained and has good rot resistance. It is easy to work with, machined well, strong for it’s weight, bends well.
The availability in the shops is usually good in spring when many people starting their outdoor construction projects in the garden.

However the quality you can buy is often poor. Most of it is „plantation wood“ and grown too quickly / soft under these conditions. You have to make a good selection and carefully sort out each piece. The better quality is fine-grained max. 20 rings to the inch (25mm) and without knotholes.

Better to choose fine grain oregon pine max. 20 rings to the inch (25mm). Pieces below with coarse grain.

In cross section make sure to choose pieces with straight grain (parallel and 90° angle to the surface) from bigger trees. Avoid the wood from small trees because the pieces have mostly curved rings (inner rings from the tree) which gives possible twisting.

Straight grain in cross section gives less distortion problems (top piece of wood above in the picture). The pieces below are from smaller trees. You can see the curved rings from the tree core.