Possible stumbling blocks:
Be sure the OUTPUT
jack can be tightened from the outside, like this one
or this other one
. Not all of them can be, and in some instances you will indeed need to stick your hand through the sound hole to secure a retaining nut on the inside. Voice of experience here: It ain't easy.
You can solder the cable from the pickup directly to the jack. I did this with two Lawrence Sound Research pickups in two Martins, thirty-plus years ago.
You can solder a mini plug (1/8" or 3mm) onto the cable from the pickup, and solder a mini jack to the terminals of the OUTPUT jack/strap pin. I did this with an Ovation, thirty years ago. This permits easy pickup swapping, should you desire to do so. But it requires sticking your hand through the sound hole to connect the pickup's mini plug to the mini jack.
You can solder a mini plug onto the cable from the pickup, solder a short cable to the OUTPUT jack, solder a mini jack onto the end of the cable, and connect that way. On one guitar, I mounted the mini jack through a small block of wood, which was then glued to a top brace near the sound hole. On another, I left the mini jack flopping around inside the guitar body, and wrapped and taped a sponge around it to keep it from rattling. Either of these methods permits quick pickup changes without reaching around blindly inside the guitar's body. Still_fiddlin's Velcro solution is more elegant than my sponge, and easier to implement than the permanently mounted mini jack. It's probably the best choice.