A D cell is 61.5mm long so you should remove 61.5mm(2.4212") of material from the loading end of a 2D mag to make it into a 1D
an 18650 is 18mmx65mm so to cut down a 2D to 18650 length you need to remove 58mm(2.2834") of material from the loading end ( 2*61.5=123 123-65=58)