Universal toothbrush cap, instructions below:

Designed to be printed without supports. Orient the piece so the open part where the toothbrush goes in faces upwards. You might have to include a small brim if your bed adhesion isn't too good.

In order for this to fit your toothbrush and not fall off, it has to be correctly scaled so that the cap fits in with slight pressure. To do this, you have to measure the width of your toothbrush head (if it has an oval shape, at it's widest point), preferably with some calipers. If you are using calipers, don't squeeze them too much :D.

This was designed for my toothbrush, with a side to side distance of 12.71 mm. So the scale you should use in your Slicer is the following:

((Your measurement in mm)/12.71)*100

Say for example your toothbrush has a width of 12.07 mm. You would scale the model in Cura by (12.07/12.71)*100 = 95%.

You should use uniform scaling, but If your toothbrush is very long you can also scale the long axis to cover the whole brush.

Printer models and filaments vary greatly, so you might have to print a couple tries anyways :(. It's less than 6 cents in material per piece though :).