Truthfully, any option that isn't a convention is vulnerable to non-republican contamination; the only difference is a matter of degree.

Might there be a better way to pre-register for this event, so as to make sure that we minimize democrat crossover?  Sure; for example, having the precinct delegates do the pre-registration via some form of door-to-door canvassing might work.  (Each county's republican executive committee could figure out for themselves how to cover precincts where they have no delegate coverage.)