Other power options and microcontrollers are available, but the DigiSpark is largely black, was best use of space for my design and have used it before. (I have a couple of trinkets kicking about but never used them).
Power-wise, i wanted to avoid LiPo, and went for 2xCR2032, but they just wouldnt have enough uumph to do the job when the brightness went up.
Measure, plan, measure, and measure again.
I tried lots of spacing options but eventually went for 25mm gaps between the slots for the leds, this allowed me enough space for batteries, micro and switch at the end.
Cut the slots for the leds.
Make sure you use a fresh blade in your scalpel/craft knife, leather is tough stuff and you want clean cuts.
Don't cut too close to the edge of the collar otherwise you will compromise its integrity.