Halcyon has a quick adjust crotch strap on some of their systems. It makes use of a buckle and a different way to thread the strap. I think the buckle could be used on any crotch strap. I don't know your access to Halcyon gear but all you would need is the buckle below:
Halcyon Slide Buckle for Quick Adjust Crotch Strap
Enhance your diving setup with Halcyon's Slide Buckle for Quick Adjust Crotch Strap. Essential for dive gear adjustments. Dive with precision!extreme-exposure.com
OK that's simple and clever too Since there's no tension on the crotch strap this seems like a goods solution too.