It's hard to tell, on Tobias Richter's renders (which were signed off by Probert) it looks more like a large circular hatch structure with obvious hull plating than a glowing crystalline object. It could be more of an aesthetic homage than an actual 23rd century style impulse defection crystal.
http://forums.scifi-meshes.com/discussion/10000650/excelsior-prototype Blasted smart phone—hard to type
The is better than most I have seen. I wonder if they used my drawings. I see some details that makes me think they did and others that make me think they didn't.
There are other Excelsior builds there as well. It is a nice site EDIT: I rather like the saucer on this one: https://www.deviantart.com/bernard-guignard/art/Excelsior-Concept-3view-684477866
I think this thread is too dormant so I've made another. https://www.trekbbs.com/threads/the-excelsior-uncovering-the-design.307623/