I almost didn’t post this because there are already a couple accessories that fill this role: Blackrapid’s FastenR and the newer, Kickstarter-funded C-Loop. But they’re all slightly different and it’s important to be able to choose from a selection rather than just going with the default.
The Sun Sniper series has been in Europe for a while and is just now getting US distribution. It’s more like the C-Loop in that it allows for independent spinning at the fastening point, but the Sun Sniper system also includes a steel-strengthened strap (protects against snatchers) and apparently a “shock absorber,” though I see no evidence of one.
Right now the standings are: Blackrapid is the cheapest and simplest, C-Loop is best if you already have a strap you can use with it, and Sun Sniper Pro is the full solution if you don’t mind paying for it. The Pro costs 69 euros, which works out to about $100. You can order it online or check for a dealer near you who might have it on sale.