Mold making question

Can you post a photo of the damage?  We might be able to suggest a fix versus making a new fin.

Yes it is possible.  However, a simpler fix might be to trace the good part onto a sheet of styrene of appropriate thickness and file or sand it to the contour.

Now;

     Here's a simple answer to your question. Take plastic sheet of the approximate base thickness. Cut larger than needed. Then carefully copy the other one. It shouldn't be that hard. You will probably need to laminate some some pieces together.

    Here is a interesting tip. Have you seen those Pin toys that you can make a copy if your hand or whatever? If you have one around, then Take the undamaged fin and fuselage rear and make a copy in the pins. Kinda like a contour guage. Then drop the new in every once in a while. You'll be able to tell if the shape is right!

I'd probably cut it out of basswood and sand the profile, seal and paint.