Mine is a square alloy welded section with the dovetail just between but just off to the side for better balance. It does have some flex only if you pull on the front or back of the scope but as I see it this should have little effect and it has no effect that i can see.
If we put the truss terminations where the dovetail is wouldn't that cause a force of pulling the back mirror support and forward ring together if there was any imbalance as the scope tilts? There is also little to no angle on the trusses to stop the forward back twisting?
The most important thing i have found is to have a very strong mirror mounting plate for the 10" Royce Conical mirror and back support ring. Mines 18mm ply with 2 layers of carbon on either side.
Here's a shot of mine