Well, you shold build an efficient reactor, but with the normal average output, and use the scrap generator. Yes, may be efficiency (aka "matter multiplier") will be less than using crazy Mk.V + breeders, but one "matter cycle" will be much shorter...
It seems more realistic... just because more effort is needed. Also I cant help but feel that the scrapbox system will get completely revamped at some point, mostly because it already feels grossly imba.