I have exactly the same problem with car I bought.
Nice cover 1.25" too short.
I tried hand stretching in front of a gas fire... no good.
Then I hung it up with two pipes wired to the holes to spread
the load and attacted two spare wheels to weigh it down. After a week
I'd got about 0.5". Then I heated it up with an electric fan heater.
Playing the heat uniformily over the thing for about half an hour still
hanging up. This gave me about 1" stretch. After a few days I got it
down and it fitted with a bit of a struggle. I left it on in the sun and
in the garage for a few days and it normalized. Now I use it
every so often just to keep it in trim.
/Steve at Guildford UK.