I like non-recessed fly seats to match up with other grip styles like cigar or Fenwick type grips at times - but there are not as many of them to pick from as 20 years ago.

Does anyone use size 16 graphite spinning seats in these applications? And any thoughts for an end cap if used up-locking?

that,s my most favored fly seat..feather-craft fly tying sells a "rubber butt cap" that is a perfect fit over the threads and really holds, no glue...you have to first turn it inside out to put it on..neat how it grabs the threads..

[www.feather-craft.com]

You can use cork, rubber or eva foam as a small fighting butt. A wood cabinet pull may even make a great butt cap.

PAC Bay makes a cap BP16B if I remember right, fits right in a 16 without mods. I also use the A7 and A8 fly reel seats for Western/Fenwick and Gordon/Ritz grips, a nice smooth transition to the cork.

Why not a Down Locking Seat? Several companies still sell USA made down lock seats. I probably install a down lock seat on 25% of my "trout" sized fly rod builds, its customer driven option. The ones I use come with a cork check and you can make a nice transition from skeleton to cork.

"And any thoughts for an end cap if used up-locking?"

Consider shortening the threaded portion of the reel seat. You don't need any more than what's necessary for fitting your reel with the locking nut fully open or "unscrewed". Then, you can add a short fighting butt/extended bumper directly to the blank. I do this on every fly rod I build.

The cap I mentioned doesn't care how you orient the reel seat.