I finally got my digital caliper for Christmas, so here's some measurements I took from the various tailcap boots that I have around.
These are some I've tried in my P2D CE. The GITD ones have posts inside that need trimming, the E1e one needs something to pad between the switch and the boot since it's pretty tall.
Here are the measurements (all in mm):
Fenix: 15.90 OD 12.0 ID 1.56 thick 5.57 tall
E1e: 15.80 OD 13.0 ID 1.80 thick 8.27 tall
Green GITD: 17.36 OD 14.0 ID 1.36 thick 8.55 tall
Blue GITD: 16.21 OD 12.6 ID 1.65 thick 6.83 tall
OD means the widest portion of the boot, the portion that's inside the tailcap. ID is the diameter of switch portion itself, what pokes through the hole in the tailcap.
The second two numbers are how thick the bit that stays inside the light is (included in the OD measurement), and the overall height of the entire boot.
As I've said in the past, the green GITD fits, but is a little larger, and the Blue GITD is almost perfect. The Blue GITD will even tailstand, but it does wobble a little.