Here is the finished motor mount and said mount installed in the body tube. I had mentioned that the retainer would interfere with the fins and have decided the fins would be offset downward. Since I didn't want to grind up the entire retainer cap, I cut some slivers of balsa from the sheet that the fins were removed from. I am currently gluing them to the part of the fin roots that will mate with the body tube. These slivers will be embedded in fillets. Since the original posting, I added a photo of the modified fins.
The next decision is whether I should reinforce the fins. Papering them might be a good idea since the rocket is heavier than stock and less fin is glued to the tube. I'm currently thinking about using a 6-7 foot 'caution tape' streamer that is looped at the rat end (like 2 streamers with both ends attached to one another). This worked very well on my 29mm Baby Bertha.
If you hover over the photo, you'll see an arrow that will let you navigate to the other photo (and the rest of my photo stream for that matter). Now, if you navigate forward you'll see the new photo of the fins.