Trunnions aren't the only way to join the ends of securing straps, but that doesn't help your issue.
If the trunnions you have/supplied are as OEM with an M5 thread in one part (clearance hole in the other) then try longer bolts than those supplied, @ least initially. This will not only help with the struggle to get the trunnions close, but will help form the shape that the straps need to get into. This will take out a lot of the natural tendency for the straps to 'spring' their ends apart. Once things have settled (helped by a Lambretta's natural tendency to vibrate) you may want to revert to the trunion bolts you currently have, or source alternatives.....
Ideally a fully threaded bolt with a couple of nuts 'locked' together, some distance below the head would be through the non-threaded trunnions. This allows a socket to comfortably fit the bolt head for 'wrenching' once the threads of bolt & trunion are engaged.
If you are struggling to locate any fasteners & they are, indeed(!) M5, let me know the current bolt lengths supplied & I will dig something out of my stock, even if it is only M5 studding.
I hope that helps
