Here are some ideas to check whether your appendage will fit through the Diamo ring.
First some measurements. The inside circumference of the Diamo ring is approximately 146 mm (5.75 inch). The ring of the Diamo is kinda egg shaped but it's a little flexible so I'm going to treat it as a circle with a diameter of 46.5 mm (1.83 inch).

 
Option 1 is to measure your appendage. There are several tutorials on how to do this on the internet, here's one with pictures (click on link)
Repeat this a couple of times. You'll see your "mileage" will vary during the day and with each measurement. When on average the circumference is below or around 146 mm (5.75 inch) you’re usually good to go, unless you have above average sized balls. Then you might want to try the following options too.

 
Option 2 is to find or buy a metal or hard plastic ring with an inside diameter of 46.5 mm (1.83 inch). These rings usually are very cheap in a hardware store. Try to get that ring around your pride and joy and wear it for a time to see if it sits comfortably. When you have trouble putting this on, search the internet for how to put on a cock-ring, check out videos as well. This video shows how I do it  (click on link).

You could also buy a metal cock-ring, prices vary from cheap to extremely expensive. I think it's nice to have and wear one anyway. You can use online calculators to find the diameter from your measured circumference to help you find the correct size. You may usually subtract a few mm from this diameter to get a snug fitting one.

 
Option 3 is creating your own temporary ring using tie wraps, zip ties, or a plastic cable (like a usb-cable) and some adhesive tape. Turn that into a ring with the circumference of the Diamo: 146 mm (5.75 inch), and try to get your crown jewels through that ring.

Hope this helps