I have a couple reasons why you would probably be better off just giving up on the idea.
IMHO, most theme clips do not show up well enough to have the desired effect. I have done rifle, tennis, golf and other clips and folks usually don't even notice until it is pointed out. The only clip that shows well is the Christian Cross.
I once tried making a cross clip for a model pen that didn't have that theme available for it. The pen kit was Tn gold. I took it to two custom jewelers, neither of which was able to successfully fasten the cross to the clip. We destroyed several clips and tiny jewelers drill bits in the process.
I don't think any glue will hold well over time.
Casting a clip is a no win proposition. The base metal for a clip needs to be spring steel. I don't think that can be cast.
On the pen I was making, I gave up and had a cross laser engraved on the upper barrel. Not the same since it doesn't show in the pocket but still a very nice pen.