This is barely an Instructable - more like a cheap kitchen hack.
That said, every year for Friendsgiving, I use our stand mixer to churn fresh butter. If you've ever tried this, you know that once the butter starts to form, the buttermilk starts to splatter everywhere.
Most folks probably wrap a bit of cling film around the bowl, or purchase one of the branded splash guards for $20.
By accident, I discovered that the lids from 10-cup Pyrex bowls are a perfect fit for the mixing bowl.
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Step 1: Find Something Round to Use As a Template
It also turns out that the ricer basket (for making super-smooth "mashed" potatoes) is almost exactly the diameter of the mixer mechanism.
It's easy to center the ricer basket over the lid, since the basket has a small hole in the center, and the lid has a small sprue from where the plastic was injected into the mold.
I then score a line around the template, and then go around that scored line 2-3 times until the plastic is cut clear-through. Any rough bits are easily trimmed away afterwards.
Step 2: ... and Viola!
Easy-peasy. Replacement lids are only $8, meaning you can buy one to cover the mixing bowl in the fridge and still save $4.
As I said, barely an Instructable. But still useful.