Put the table tennis ball half in to the cap and using a sharp pencil (for accuracy) draw a line round the table tennis ball so that you can cut it to fit the cap. If you do not have a table tennis ball and you are using the underarm deodorant then take extreme care when cutting it. If any of these are not to hand, as alternatives, try: An under-arm deodorant ball roller for the eye (take care hack sawing it in half, they are strong) An old handbag or laptop bag strap (check with owner first!
Prise the ball holder out of the bottle first, then pop the ball out.
Now offer the pieces up together again and trim off any parts that look messy or don't quite fit.
Becuase this is a project made out of junk, you might have to be inventive, I have given a few alternatives.
(If you are careful with the knife you will not need to do this)Just be careful of your fingers.
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You could even finish off the hole with fine sandpaper (or a file) to make it nice and smooth, but if you do this, don't smooth the top of the lid with the sandpaper and you will only scratch it up.
To get a nice even cut don't aim to cut the hole out all in one go, cut nearly to the edge and then carve away at the hole carefully making the hole nice and even.
Check that you have a nice angle between the two strap parts.
Take the ball out of the cap and cut to the line using scissorsCut them using scissors (the best way to cut leather, safer and neater than a craft knife) (also trim the straps at the EYE END, ie with a slight curve as you marked previously, and in this case cut off the buckle.
It's easy to get them out of the bottle (do it with a blunt knife).
Most caps have alittle rim in the inside to help with the sealing, it's probably easiest if you cut up to this rim.