Cheap is going to shrink. In the sun vinyl is hard to get in a quality which is NOT going to just shrivel up into a mess with a few weeks sun beating down on it.
If you buy some and are not certain, just leave a piece out on your dash in the sun to bake for a few weeks. You will find out pretty soon if it is any good. Measure it and then see if it is the same size three weeks later. Summer is perfect for this. the interior gets to 130 F easy... Baking in the sun all day.