I have the same boat (IP380-89) and I bought a Kato radar mast mount. I also antagonized over this same question for a long time. I looked at IP380 photos and read everything I could find on the subject. In the end I opted for the solution sent to me by 'Island Yachting Centre' in Palmetto Florida - see below.
"The radar goes under the spreaders where the steaming light is. You actually use the hole of the steaming light to pull the radar cable up the mast and then put the steaming light under the radar."