Completely disassemble the TC to avoid chips inside.
Drill to cut away the optics, like a Swiss cheese, first with drill of 2mm close to each other, then remove the in-between part drilling with a 3mm.
Watch out for the CPU print board while working on it.
Remove all the rough edges with a half round Needle file.
Apply matt black paint.
Reassemble.
Just finished the two very nice extension tubes for Frank; TC-14E AF-I version and TC-E20E II
Internal flocking is best applied during assembly for the 20, the 14 just get a bit after assembly.
Also remove the rightmost contact on the 'female' contact block to keep focus speed fast for the 20
Warning:When removing the 'female' contact block take extreme care!
Especially the 14 is not advisable to remove, better to leave it on and shield it from metal parts while drilling.
The 20 slides out with great care,,,
Reason: The flexible print board are soldered onto the block in situ...
Don't ask,,,