Most bassoonists will agree that low D is one of the worst notes on any bassoon. It's unstable, unfocused and easy to play out of tune.
Alan Fox, of Fox Products Corporation refers to low D as the "Sacrificial Lamb" of the Bassoon. The placement and size of the tone hole are not optimal for low D itself, but, since many other notes on the bassoon make their acoustical home partly in that tone hole, the placement and tone hole size are a compromise.
I have recently come upon a cheap way of improving stability, pitch and focus of low D that anyone with a ruler, scissors and a small screwdriver can implement.
After removing keys around the low D tone hole, find an ordinary soda can or the like:
Empty it and cut a strip out of it lengthwise:
The circumference of my low D comes out to about 1 3/4" or just under 45mm. Cut the length a bit longer than this so you can trim it to fit snugly in the tone hole.
Curl the strip with your fingers to approximate the circumference of the tone hole before fitting it in. The aluminum is thin enough to be easily manipulated, yet it will hold its shape and stick securely to the sides of the tone hole without the need for an adhesive.
Place the strip in the tone hole with a tweezers, putting the strip in so it sits just below the lip of the tone hole. A certain amount of the width will protrude into the bore -- that's on purpose!
Thanks to James Roberson for these instructions and to Carl Sawicki for his idea.