Adding weight for hangboarding

What's a good vest for adding weight hangboarding? Or is there any kind of a belt for this instead? My worry is that the weight of a vest pushing down or your shoulders would add artificial support thus leaving shoulder stabilizing muscles deficient. I don't know if that's a valid concern or not. Obviously I don't know much about this stuff but I've been using a hang board close to daily for a while now and I'm getting up there in pull-up reps. Doing 20+ pull-ups is painful no matter how much you do them. I'd like to add enough weight to get back down to the failure at 8-12 reps that most fitness gurus think is ideal for building muscle. Any advice is appreciated. 

I (and most people, I think) use a harness with weight hung from the belay loop,

Yeah most people use a harness, sometimes with the leg loops removed and clip weight to that.

+1 on the harness.

And doing pullups on the hangboard is not the primary way of using a hangbord. You might consider doing hangs on smaller holds, instead of doing lots of pullups on bigger holds.

And if you still want to do pullups then IMO there is more benefit in doing fewer pullups with heavier weight (e.g. a weight that would get you to max effort in 4-5 pullups, and doing 3 sets of that, with rests in between), vs doing 8-12 pullups with the weight that you can handle for dozen pullups.

Cut off the leg loops of an old harness, makes it more comfortable and similar to a weightlifting belt.

Just buy a dip belt from the start. Best $10 I ever gave Amazon. Harness sucks, only tolerable up to ~20 kg. I started to worry that my internal organs would pop.

You are correct that 8-12 reps is the standard hypertrophy range. You may benefit from some training in the 1-4 rep zone though to develop max strength, which is great for climbers because it comes with no weight gain penalty. The gains come from developing your neurological system.

The weight belt is preferred, as the weight we have is more centered around the hips and not the chest (backpack, vest). For hitting your rep range, you could try more difficult grip positions. Building muscle isn't the goal for climbers, it's toughening the musculoskeletal system to take weight.