This is a parametric razor cover that should fit most disposable razors.  I made a few covers in the past for specific razor models and often got comments that they were slightly too big or too small - which is due to the manufacturers changing the heads from year to year.  So now you can measure the head of your razor and make a cover that fits exactly.

I would recommend adding .3 to .5 millimeter to any measurement for the razor.  If the width is 42mm, enter it in as 42.4mm - it's good to have a little extra room around the head of the razor.

I've included a couple of example models that I made from this so that you can see what they look like in the 3D viewer