An 8ft 2 x 4 will provide two 45" tray sides or three 29" tray ends; one GBG needs two of each. Select for straight without wane or big knots. Some warp is ok in 29" pieces. The cuts provide 6" or 9" (-kerf) of scrap ; use this leeway to avoid big knots and split or uneven mill ends. Ends must be 90º, or the tray will warp or rack. If it does, use builders' shims to square up the joints.
A 12-ft common pine 2x2 will provide 12" and 60" masts for two GBGs. Select for straight with no big knots.
A 4 x 8ft exterior-grade 1/4" plywood sheet will provide three 32" x 45" tray floors. From one 3" x 32" scrap, cut 24 "wood staples" 1-1/2" square. Save the two remaining 3" x 32" scraps. With two more from the next sheet, you can make a tray warp gauge.
A 4 x 8ft exterior-grade (ACX preferably) 1/2" plywood sheet will provide enough trapezoidal gussets and propeller arms, and rectangular tray feet, for 16 GBGs, plus extra feet. Use these cutting plans:
When the drum is re-erected, the keeper has to slide easily into the 2-1/2" length, to lock it. To prevent sharp-cornered pipe ends from interfering, sand or rasp to round off the outside corners of the keeper (both ends) and the inside beveled end of the long axle.