the one thing that makes it believeable or not is outside reflections on the glass. now im assuming you already created your reflections of your object on the inside back walls when you rendered out the 3D stuff.
to get the look that real glass in front of the object is only a small task:
- position your object where you would like it in the shot.
- create a rotoshape that matches the the shape of your 3D object (or if you have the alpha rendered out use that).
- use that rotoshape or alpha channel render as a mask on top of your glass box image (or video).
- have a duplicate of your glass image ready to use as a background layer.
- attach a multilayer node to your glass box image and have it be your bottom layer.
- put your 3D object in the second input just as on "over" comp layer.
- give your masked node from earlier a luma key and make only bright areas visible.
- direct the output of the luma key node to the 3rd input of the Multilayer node.
thats pretty much how you do it in a nut shell. the rest is just fine tune adjustments till you think you have it right. if you think about real world glass and when you look through it, you see a shine on the glass which is pretty much light reflections from the same side you are viewing from. if you can isolate those specific areas of shine or reflection, you can place them back on top to make your 3D object seem as though it is behind that glass.