How to fixate RJ-45 double coupling
You want to take a look at what is called a Keystone Wall Plate. These have rectangular holes that compatible RJ-45 jacks and adapters can slide in and snap into place. The picture of your adapter looks like it is a compatible type. Here is what one of these looks like (front and back):

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You could take careful measurements off one of these wall plates and come up with a suitable 3D model for the hole and retainer edges that you could factor into your 3D printer model.
Alternatively, once you do some research on various types of products that incorporate Keystone modular mount holes, you may decide to purchase some ready made item. Such item could be used as intended or you could take a DIY approach and modify the item to fit your application. Check out the following link for an example 3D printer project.
https://www.thingiverse.com/thing:4302166

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