Here's my GB2 stethoscope and some of the steps I took creating it.
If you want to make one yourself, you'll need a black stethoscope, a vintage wire-frame headset, and something cone shaped.
Gently pull the drum off of the hose attached to the stethoscope.
Then chop that stem off! (But save the stem. you'll need it later)
For the cone shape I started out with a regular funnel but I wasn't getting the desired shape. But I found this funnel that worked really well. You can find it at a Michael's craft store in the baking department.
Chop down that cone too! Before chopping the cone, you can place the drum into it and trace inside to have a decent indication of where you want to cut. Feel free to put it into boiling hat water if you'd like to soften it a little.
You want them to fit together like this once you've shaved the cone down.
You may want to bevel the stem before going to the next step. So when you use epoxy it will not slide out.
Paint the cone and the drum then epoxy the stem into the top.
find yourself a ring to place at the base of the stem. I found this in my box of parts.
Paint the ring black if it isn't already.