"Tanner-'op" wrote in message
...

[1] Can be cut back at the workshop when you are not busy and stored
ready for use, and one size fits all for most jobs (on door and window
frames that is) - with the added bonus that they can be used for a
multitude of other purposes when "caught out" or in an emergency.

One size fits all?.. not a chance.
If you are making them two foot long and only 3 mm thick maybe.
If you try and fill a small gap with them you only get support on the edges
and then you can't get a secure fixing.
You need several thicknesses to do the job properly.
You obviously lack experience using them.

Something a flimsy plastic peg cannot do!

Try it and see.
Almost any tough plastic can make a packing piece.
I have used Formica plastic off cuts in the past.