If you frequently have to tighten or remove socket nuts from Eurorack modules or semi-modular synths, the best choice would be the excellent Exploding Shed nut tool set.
If you only need a tool for Thonkiconn nuts and have access to a 3D printer, or if you're outside the EU (Exploding Shed only ship within the EU), here's a free STL file for a DIY nut tool that you can print out: Download
This has been tested with PLA, 0.1 mm layer height, 3 wall lines, 10% infill.
All you need is some superglue and 2 nails (1.8 mm width which is about 2D or gauge 15 in the US). Put a bit of superglue on the shafts. If they're long enough, insert the nails from the top. Otherwise, cut off the head and insert from the bottom. Insert tool onto a Thonkiconn with the nut already in place. Make sure the nails sit in the nut slots (to adjust the position of the nails). Wait to dry.
If you want to use nails of a different shaft width, the download includes the OpenSCAD source code, so you can easily change the hole diameter and generate your custom STL file for printing.