I think you've answered the question yourself to be honest.
An engineering employer is looking for a good degree grade (that used to be a 2:2 but nowadays with grade creep increasingly it means a 2:1, which is the "new 2:2), and good experience.
So, if you are lacking in both departments you need to try and achieve an improvement in both. You can't improve your original degree grade, but you might manage to get an MSc or Mphil, which should pretty much wipe that out.
As for experience, it's out there to be grabbed. If nobody will pay you for it, look at what you can do unpaid - help the local PFA stut members building aeroplanes and designing mods, go and try and help with a restoration project somewhere like Duxford or Old Warden, get involved in one of the hi-tec amateur groups like Aspirespace and get professional standard experience there.
But, lets be honest, with a 3rd and no experience, your chances of getting a job until you've improved on that are pretty slim.
G