I had the same idea. Got my spare spring from J. Finn. Now I thinking of trying the shackle idea.
First Idea was to drill a hole thru the spring, put a plate on top and on the bottom of the cross member and use a piece of threaded stock thru.Then use a lock nut on the top and one on the bottom. (I am not sure of drilling a hole in the spring)
The only other way to do it would to make two L shaped pieces out of threaded stock and hook them over the spring, drill two holes in the cross member. Run a plate under the cross member with two
holes lined up and put nuts on them .
What do you think?