Panel shims

Can affect the rider's pelvis tilt, refines the twist, and slightly deepens the seat. It goes under the front of the seat, with the wide side toward the pommel and the pointy side toward the rider. It can be moved forward/back for a greater effect.
Riders who have a low pelvic floor may not like the bolster. For those who are looking at the product to tilt the pelvis, one of the other rider balancing aids will have a great effect without discomfort.
To install, release the velcro under the front center of the seat, then tug the seat back over the front dees. Place the bolster and press the seat back into position, making sure all is still symmetrical. Replace the front velcro tab and push the seat back over the front dees. This last part can be difficult, especially when the saddle is new - to make it easier, pull some twine or a shoelace through the dee to give you more leverage.

If panel shims aren't enough or you have closed panels, it's also possible to shim in the pads (although the pocket shims for close panels may be a better option for both cases). You want to make sure you are still using the foam pad inserts so you don't put a transition next to the horse, plus they'll stay in place better.
One advantage of the pad shims is that they are easy to put in place while the saddle is on the horse (not girthed) and can easily be removed if one horse needs shims and another does not.

For those who want a cheaper option or more ability to customize, I've made some less polished seat reducers from wool felt. And here's a hint - if you have similar material laying around, you can make your own or even do other customizations to the fit of your saddle.
These are made from 1/4" thick each and can be used with 1-3 (maybe 4 if you adjust the position of your seat back on the base). Typical use is 2-3 for ~1/2" to 3/4" smaller seat

Panels for Ghost saddles - and can work for other saddles with loop-side velcro underneath. Price is for a pair of panels or the complete western skirt with integrated panels.
New option - XL velcro opening panels, great for those really wide horses, but may also provide more stability for slab sided horses. (NOT available - or needed - with Thick insert option) Panels for 16" seat run just over 20", for 17" seat is about 21", for 18" is a bit over 22". Important - the XL panels are 7" wide so you need to make sure your pad structure supports that! See more details below.