I was... derailed from my plans for some time. but...
To answer some questions, the "best" tools I personally have used are:
1) Supple Nips / Supple Cups -
One's sold more for inverted nipples as a more straight laced outlet, the other is more oriented to the fetish scene. Both are the same product.
They are a firm rubbery substance (food grade silicone actually), generally clear, and generally in 4 sizes (there used to be a 5th larger size, but its no longer made).
They are tricky to apply well, since you have to squeeze out all of the air from the cups, apply it, and then very carefully let it suck nipple flesh into it without letting it break it's suction seal. This is impossible without lubrication, but with lube, and some care/practice, it can be done.
Once in place, they are the best option in terms of size, comfort, low risk of blistering, and staying in place for long term use.
I use the size 4 (wish they still made the size 5!). Size 4 is 3/4" inside x about 1 to 1&1/4 inches long (about the size of a thimble made for a thumb).
2) Avent Niplette -
These are hard plastic nipple cups with a small tube for removing the air. They come with a little syringe for removing the air via the tube.
These are great for being fairly easy to apply with a little practice (except the syringe gets wonky after some use).
And they are great for staying in place under a bra fairly well for long term use.
If the vacuum is too great, you will tend to blister at the spot where the tube diverts away from the cup itself, so it requires some practice to get right.
I no longer use these, they're too small for me anymore. Their size is about 1/2 inch opening, by about 3/4 inch length (about the size of a thimble made for a small-ish finger).
Both work "best" when used with a cheap water based sex lube. For this purpose, you actually want a lousy sex low quality lube. Less than ideal for sex, but it will dry down to a sticky residue, which makes a lasting seal, and helps significantly with keeping them in place.
These are preferred because you can wear them for hours and hours, or potentially go to bed with them on. I go all night with them on, and i only knock one loose overnight about 1/3 of the time. And body modding using push-pull type methods depend more on the 'time' involved, rather than the amount of force. Think braces for teeth, or creating an ear loop with increasingly larger plugs, african neck elongation, or corset training... slow & steady progress.
Since I've kinda re-started working on my goals (and have somewhat more flushed out goals and plans)... I've been looking for some additional tools for nipple enlargement with more size than the size 4 supple nips (more length, width, or both), but i haven't found anything super useful yet that has the right amount of suction force and a good fit that stays in place well.