Rhombi-Cube-Octa Dome [Simplified Version] as a 21 ft 2 x 6 frame.

 
Notice on this illustration the equilateral triangle pyramids are removed.
This 21 ft diameter size is optimal for efficient covering with the cover panels made from 4 x 8 ft sheets.
The less expensive 92-5/8" lumber can be used; no 8 ft boards required.
Half of the cover panels will be full triangles and half will be "X1" right triangles.
For a greenhouse, a simple seam cover can be used on these right triangle panels, or make X1 struts for these on half of the dome only - more light inside.
If using board panels for dome covering, the X1 struts should be used, preferrably 2 x 4 or 2 x 6.



All the parts of the 2 x 6 RCO-S-X1 Dome, with equilateral triangle pyramids, and X1 struts shown in green.


Next, a Cupola and double door are added >>