I came across the double D-ring buckle on my new motorcycle helmet. Soon after I learned on the internet that this kind of buckle was used in ancient times already. Even more importantly, until today it is one of the toughest buckle mechanism out there. 
 

That got me interested and I started to investigate how much load a 3D printed double D-ring buckle can handle. The short answer is: Much more than you will need if you use it on your clothing. 

Both parts in this publication are load tested at more that 120kg pulling force on the strap. (That is no guarantee of course, it is just the result of my testing). The difference between the parts is how sharp the relevant edges are. 

I do recommend to use the part with the chamfered (softer) edges. Even this part did not let the strap slip in any of my load tests. With this one it is easily possible to pull the strap tight. The part with hard edges is for you if you are completely paranoid about loosing your pants at work. It holds the strap (or belt) very strongly. But it is actually quite a bit harder to pull it tight. And once it is tight, it is not easy to loosen under load, unless you mange to get the load off for a moment.

Printing

It is actually very simple to print these parts. Just print them flat on the bed with 100% infill. Both PLA and PETG have shown to work well and have passed the load tests as stated above.

What you need in addition

To form a very strong loop of you strap though your D-rings, you need the strap tensioner that was also part of my load testing. Also this part has passed the 120kg load test without showing any sign of damage and of course without letting the strap slip trough.
https://www.prusaprinters.org/prints/121221-strap-tensioner-load-optimized

I use this buckle on the easy-to-get one inch wide nylon straps that are used on backpacks, traveling bags, etc. It is sold online for well below 1€ per meter and comes in many nice colors. I use such a strap as a belt on my motorcycle pants. Not using a metal belt buckle prevents my tank from getting scratched. 

Disclaimer

Although I am very confident with this kind of buckle, you use it completely at your own risk.