Wrong knot. Square knot (AKA "reef knot" for sailors or "package knot" to many old folks) should not be used to fasten ends together . I wish we would quit teaching it as a first knot to Scouts. It is actually a fairly specialized knot that is a poor choice for general applications. Fine knot for securing reefed sails (I guess - not a sailor) or starting off western-style spar lashings...

For what you are doing I also would not use the sheet bend (beckett) family, although they excel at many general things involving joining various ends together and some specialized uses.

Instead, I suggest that you use a fisherman's or (nicer looking) a double fisherman's - they carry alternative names, depending on user and region. In small laid cord that will be a permanent knot unless you are really desperate to untie it. If using really slippery stuff like Spectra or monfilament, stick to the double version, but if you learn to tie the single version first, it is easier to learn the double IMHO. Once snug, you may safely trim the tag ends quite short - as close as 3 cord diameters in accessory cord, although I err a bit on the side of caution. Looks 'factory made" in small accessory cord.

Get some 2mm accessory cord and it will hold up much better in your pocket - the laid nylon gets all pilled and fuzzy after several months use. I have plenty of uses for the laid nylon twine - not knocking it, especially as cheap as it is.

The double fisherman's is what I always use for your application - and some similar "large" items with 7mm - 8mm accesssory cord.

Let me know if you cannot find an illustration of how to tie a double fisherman's.

Regards,

Tom