Since I just finished my Phantom TKL a few days ago, let me offer a few thoughts. Building the board was pretty straightforward. Almost all of the problems I had to solve were related to case fit and the USB cable.
How close down on the PCB did you manage to get your Teensy? I only put a piece of rubber electrical tape between for insulation. Later I wished I'd used thinner vinyl electrical tape. My case really had no clearance to spare in that area!
A Filco case positions the indicator LEDs 8mm up above the PCB, and normally you would have a plastic standoff to fill that gap. I tried to improvise one from cardboard, but that didn't work at all. Later I got some SIP sockets and soldered them into the LED positions. That allows me to change out the LEDs easily at any time, and they are tall enough that I can get away without any standoffs.
I'm not sure what case you plan on using, but just because the Phantom is designed to fit in a Filco case and a case may be designed to accept a Filco board, it doesn't mean they'll fit together—without a lot of grinding, anyhow. My first choice of an Iqunix Lambo TKL case would have required more modding than I wanted to get into. I ended up using a TEX TKL case. It doesn't really provide any mounting support for the PCB, though. My original plan was to improvise some support around the edges of the case using Sugru, but then I realized I had a sheet of ⅛" thick Sorbothane® in the closet, so I cut a piece to fill the bottom of the case. It was a very tight fit, but it supports the PCB, and everything is so snug in the case that nothing can shift at all. (I really thought I'd have to do at least a little grinding where the Teeny's USB connector sits, but when I tightened the screws down everything somehow went together.)
The standard Filco USB has a funky not-USB connector on the end, where my Teensy needed a USB Mini connector. I discarded that cable and got a nice, braided USB Mini cable. It should be possible to rig some kind of adapter from the Teensy to the case opening and have a detachable cable, but I was too lazy. I ground down the connector housing with a rasp until it was flush with the Teensy (and would fit into my case) and then glued the cable permanently into the case and filled the opening with Sugru, and I think it'll be OK.
Also, this was my only experience working with Costar type stabilizers. They can be sweet when they are lubed and adjusted right, but they can be cranky too. I had to shave down some of the inserts to get them moving freely in their slots.
