Here are the antenna's SWR and radiation pattern plots as seen with 4NEC2 ...
The gain, F/B, and SWR curves are all fairly 'flat' to provide consistent performance across the 8m Band and to be tolerant of minor contruction errors.
The diagram below shows the tapered element lengths and spacings. Remember to add about 100mm to all of the 3/8" diameter tip lengths to slide inside the ends of the center sections ...
The tapered element joins were clamped with small SS hose clips (cut slots in the ends of the 1/2' diameter center sections for a compression joint). The element tips could have been permanently fixed in place with small self-tapping screws or rivets.
The parasitic elements were mounted on the 30mm x 30mm square boom using 'plastic' hydraulic pipe clamps (Stauff clamps). These clamps space and insulate the elements from the metal boom so no 'correction factor' is required.
The split dipole driven element was mounted on a 250mm x 50mm plate of 1/2" thick PVC. HDPE (white 'plastic' kitchen cutting board) would also work. The gap between the inner ends of the split dipole was about 15 or 20mm ... not measured because it is not critical.
The Yagi's feed point impedance is 50 Ohms for connection to a 50 Ohm coax feedline via a 1:1 balun. I made a W2DU style balun with about a dozen 10mm long Laird 28B ferrite cores that happened to be on hand and slid nicely over a piece of RG-58 sized coax. Similar sized '73' material ferrite cores would work just as well. The measured impedance of the ferrite choke balun broadly peaked at about 45MHz ... close enough. The ferrites were fixed/waterproofed with a length of glue lined heat shrink tubing.
Finally, if required, adjust/trim the DE length for best SWR at 40.680 MHz (mine was OK 'as is').
The completed Yagi ...
Note that the 1/2" diameter center section of my Yagi's driven element is considerably longer than shown in the diagram above. That is because I used an old 1/2" dia 6m Yagi driven element that was on hand ... and rather than cut it to 1200mm long, the 3/8" dia tip lengths were changed instead. In my case the DE has a 2850mm long 1/2" dia center section with 340mm (exposed length) 3/8" dia tips at each end.