Lake, the MKXII ( which is what the Seafire MKXV fundementally was) was built on the MKV (early MKXII) and MKVIII frames so the BT Spit would be a great starting point. Just remember that length of the Spitfire mkVIII was 11.5 inches less than the Seafire XV due to the Gryphon engine. The Spitfire MKXII was some five inches less than the Seafire MKXV so the difference would probably be in the rudder/arrestor area. The Seafire MKXV was built with both types of arrestor hooks, the V type and later the Superior sting type and all had the retractable tailwheel. Hey Chad, the last 30 MKXV Seafires were built with the bubble canopy. The main external distinction between the XVII and these late XV's were the strengthened main undercarriage so it would be as easy to build either variant.
Good luck
Paul