Blues Junior / Jensen Blackbird 12" does it fit ?

[M Fowler]

The stock Fender Blues Junior has a lot of room around the speaker.

I recently rebuilt a Blues Jr to Matchless Lightning using much larger transformers and no problems.

I have two stock Blues Jr cabs with custom chassis sitting here and I know I could put in 6L6's and large transformers and not get close to the bells or frame of the speaker.

Your custom pine cab is probably larger than a stock Fender BJ.

Mark