Materials needed: a duo fiber brush of your choice and a small rubber band:
Place the rubber band like so around the back end of the brush, and gently push it forward.
Stop pushing it forward about 1/3 way up the bristles. This location resembles the diameter of the MAC 188. It ain't that pretty to look at, but who cares? I can use that money for a certain ombre blush I've been lemming instead!
Now for the review: the smaller diameter of this brush is absolutely ideal for precise placement of blush and for those with smaller cheekbones. This brush is also ideal if you use multiple blushes at different points of your face. However, since this brush is extremely soft, you will have to build up color with multiple layers, especially when working with mineralized blushes. This brush would also work perfectly with highly-pigmented cream blushes.
Please note: If you take off the rubber band, you will see a dent in the brush similar to when you have a dent in your hair from a scrunchie. So make sure you are okay with that before you proceed! I have 2 stippler brushes, so I keep one in its original condition, and the second one has this rubber band on it at all times.
I hope this DIY inspires you to turn your spare stipplers into a MAC 188 dupe. After I started using dense flat-top brushes for applying foundations, my poor duo fiber stippler brushes fell by the wayside, so I'm very happy that they've been given a new purpose. :)