Not sure I missed doing this one ages ago, but better late than never. The Bassballs is a classic envelope filter that's been a staple of Electro Harmonix's line up for a long time. Designed for bass, but works great on guitar as well. Trimmers are very dynamic and might be fun as external pots. Also if it's super loud, adding a volume pot at the output is super easy—remove the 47k resistor off the Out pad, wire the Out pad to lug 3 of an A100k pot, lug 1 to ground, and lug 2 becomes the new circuit output. Here's the schematic for reference. Should fit in a 1590B, though it'll be a tight squeeze.