Depending on the weight of the prism, you might be able to get away with a PCI slot spacer that is either rivited or epoxied to the internal side of your case.
having a rigid extension out from the back of your case would not be so bad, as long as it was painted to camoflauge with the background. Think 2D like paper, turned one way, the paper is visible, look at it's edge, and you cant see it.
You could also get a clear acrylic rod or a tube, and fuse that to the prism.
There are lots of ways that you could suspend the item. Just make sure that the supports are X-Y-Z supported. IOW, if you just had it on a pedistal, if you tipped the unit on it's side for shipping, it will flex the pedistal and possibly break it's adhesion point.
Definately a little bit of designing to do and experiment with.
