Hey DaveySew, have a look at my albums (link below).
I moved the seat lock to the back mudguard as per Triumphnet...
Seat Lock Mod
Then I made myself a pair of hooks from Duralumin that attach to the bike where the two big allen bolts hold the bodywork to the subframe near the battery.
The hooks take the d-ring and then the seat locks down. They are tight enough that you can't really get to the strap to cut it, and the hook cant be bent as it is against the underside of the plastic tray of the seat.
I also did a short thread on them
Helmet Hooks for upto '04 Sprints
Hope this gives you a cheaper option. Catch me after 14 June, and I could even make you a pair!
The only down side is that they work with the D-ring type. I don't have the other sort to play with
Rexx