Another inexpensive alternative to the Spillmaster is the spill-proof paint cup available from Lakeshore Learning (a nationwide supplier of school supplies). Here's the link:
https://www.lakeshorelearning.com/product/productDet.jsp?productItemID=1,689,949,371,896,043&ASSORTMENT<>ast_id=1408474395181113&bmUID=1519371850665
They're $12.99 + about $5.00 shipping for a set of 10 (various colors of lids), which amounts to $1.80 apiece. You can get individual colors for $1.49 each (+ shipping) or see-thru lids for $1.69 each (+ shipping). Lakeshore also has stores in a number of cities. If you're near one, you can save the shipping cost.
As shown in the photo, you can get an acid brush that's a perfect size for one of these, allowing the cover to be closed with the brush completely inside. The brushes are available in 3-packs from Home Depot. Regular length acid brushes will work, too, and there are features on the lid to keep the brush from rolling away.
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As Hozzie mentioned, similar paint cups/pots are available from Amazon (and various other vendors). I've tried some of the others and prefer the Lakeshore cups for two reasons - (1) Most other cups have snap-on lids, which aren't as secure as well as Lakeshore's screw-on. (2) Many of the cups have "living hinge" covers, and the hinge is likely to stop living when exposed to machining fluids. The Lakeshore cup has a molded pin hinge, so I think it's more likely to survive in the shop.
If you want to "gild the lily," you can glue a large washer to the bottom of the cup with "Go2" glue, as shown in the first photo. Or you can drop a washer and/or a magnet inside. The second photo shows some wrought iron rings I found locally at Industrial Metals Supply. The gap in the ring can be adjusted to give a nice, snug fit in the bottom of the cup.
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