This sponge holder can be mounted inside an under-sink cabinet, so you can keep your countertops clean. It has a drip area and is well-ventilated. It had holes for mounting with screws.

The file here is for the holder in the right of the photo; the left one was a prototype (I decided it was too narrow, and the screw holes were a bit small). For the record, the other side of the cabinet door looks way better than the inside shown here! Yikes!

It was created in OpenSCAD; you can customize the size, hole size, etc. fairly easily. The code isn't super clean, but should be easy to follow if you have any OpenSCAD experience.