I'm looking at installing diagonal bracing to my roof trusses as described on page 77 of the document at http://www.dwbgroup.co.uk/files/8013/5284/7141/Gang_Nail_Trussed_Rafter_Manual.pdf but am unable to find a supplier of the "22 x 97 x 600mm long timber splice plate" which is specified as a binder to protect the braces where they cross each other. The only similar product I've been able to find locally is 25 x 110 x 335mm. As this is not being used to join the timber but just (as far as I can see from the available documentation) protect it from damage caused by it moving against the brace running in the opposite direction, would it be possible to cut this down to size in the directions where it is too large using a grinder, and use 2 pairs of them rather than just a single pair to extend the length?
If the purpose of the plans is to just protect against damage, I think your plan is probably fine (you might not even have to cut them down to size, I don't see the problem with it overhanging a little bit).
However, it's not clear to me based on that document exactly what the purpose of the splice plates are. To me it sounds like the splice plates are on the outside faces of the braces, nailed through from both sides.
I would call the truss manufacturer to find out specifically what their bracing requirements are.