I would use a very small board. Nothing large will work.
Cyborg/Droid '06/'07 LED boards are very small. I'm using the bluefork set up mentioned above. You still need to mill an eye wire channel into the bluefork frame where the connector addition is added to mount the frame.
The DNA boards from Curt on inceptionforums is probably the best choice, but they aren't finished and he went MIA due to some new job he got.
The scenario Dreams T-Board will also work, as it's very small. Has a lot of wires though, especially when mushed into a PMR UL frame.
You can use an infinity legend frame with a legend board I believe as well.
You can also use a Quest frame/board combo.
Either way, that's about that. Other custom milled frames have used OLED Tadao boards, but I would stay far far away from those as the screens burn out.
I have the link saved somewhere with detailed pictures of A-Tach-One and what he went through to make the PMR frame fit the solenoid/board. I'll send it over to you if I can find it.
There's the link. He shows the steps and details what you need to do to a PMR UL frame to make it work.