I use Boingos as well as the ultra-common Dritz diaper pins (the baby kind). There are some decent adult-sized diaper pins out there, but most (like the Baby Pants ones) are garbage -- taper is too steep. Plus, I've found that a single Dritz pin per side is just fine for me.
I have a 38" waist, and while I've heard of folks my size using Snappis, I personally find them wayyyy too small. Basically, to use a Snappi as an adult, you have to oversize your diaper so that the back corners are very close together in front when you pull them forward. Yes, Snappis are rubber, but they're not very stretchy!
I have the same waist size as you and use the size 2 Snappi fasteners. I tugged at them for awhile getting them to get a little stretchier. They work quite well for me. How big are your pre-folds? I use XL Baby-Pants cotton twill, but also stuff them enormously with a bath towel and a dozen baby pre-folds. They work and fit great (a bit tight but it is strong).