Why not buy one? Most brush cleaners I've seen only have about an inch of room underneath the grill, and then about five or six inches of unusable space up top. That's stupid. I want to fill that nasty jar full of toxic gunk and not have to get myself a new one for several years. I need, like, four inches.
Don't, Dave. Don't.
I used an old mayo jar the first time around and it was a headache to get the grill in. I recommend you get yourself a sturdy tupperware with a screw lid. No snap lids and NO GLASS. This isn't a BPA-free snack hour at the local Pre-K. You want something sturdy and smash-proof.
Cut out a square of 1/4 inch avian steel mesh.
Mark off the height of your "shelf" on an old plastic bottle.
Cut up that bottle and stick it in your rinse jar. I like to cut out a couple little teeth that will stick up through the grill to keep it from shifting around too much.
Now push the grill down over top. Fill up with mineral spirits to about half an inch above the lowest part of the grill and get ready to rock and roll. Stay sexy, readers.
Addendum: Now I know you all collectively lost your heads when we discussed how to make panels but didn't give a detailed list of every Dibond provider in North America so that you could all quickly look up your nearest provider. Avian mesh is also challenging to find. You will want to find someone who breeds birds (Craigslist is a good place to start). These handy folk make their own cages and have scrap avian mesh lying around, theoretically. Or you could get some half-inch chicken wire, which is probably easier to find, but I like the quarter inch aviary stuff pictured above best. Or, cannibalize an old mesh strainer (50 cents at Goodwill) or ask your local hardware store if they have something appropriate. I don't know. You're all brilliant and smart and resourceful. When you figure something out, share the results in the comments so that others can benefit from your smarts.
And, heeey, did you notice we now have bylines? Thanks Amanda for suggesting that. When you've been sharing a toothbrush with someone for seven years you forget that other people actually view you as separate beings, and that they might want to know which half of the borg is narrating a post.