I am not sure, what the size of the heatshrink is. I took my keyboard to the hardware store, they had wheels (you payed by length) with different cable sleeve sizes (and colours). My Choc Mini has 4mm cable so I got a 3mm for it to fit on nicely. The shopkeeper supplied the heat shirk suitable for my cable.
If any of you want to make one, take the cable with you, a visual reference of the cable is the best way to make sure that you get the right size. I'
P.S. If any of you are attempting to do this in a keyboard with a non-detachable cable (like me) please share how you made a nice finish in the end of the cable that touches the keyboard. I have tried heat shrink and it looks horrible