I thought that the thickness of the wood was an essential part of the aesthetic of this design. Thin the timber, and its visual appeal changes.
When I lived in Boston the most popular timbers (read cheapest) were Eastern White Pine (Pinus strobus - density = 0.400 and Western White Pine (Pinus monticola - density = 0.43) and I presume that these are the intended species for the chair/bed. Both of these are considerably lighter than radiata pine (Pinus radiata - density = 0.51). Changing the wood from EWP to radiata, leaving all dimensions unchanged, would increase the weight by 27.5%; changing to an Oz hard wood could double the weight.
Thus I would suggest that it is very important to this design to use very light weight timbers. The only one's that I can think of that are reasonably available and affordable are:
- western red cedar - Thuja plicata - density = 0.37
- paulonia - Paulownia tomentosa - density = 0.28
I saw WRC at a local suppliers last week in sizes up to 200 x 45 mm.