...every time I lift my case with the laptop in it the lid opens and therefore my laptop resumes from sleep.
If I'm understanding your problem, you want to keep the lid secure while the laptop is in transit so the micro switch that controls sleep mode isn't activated. What about two small hook and loop tabs at the front corners that would keep the lid closed? I did that to a laptop I had with the same problem and it worked great.
Or a heavy duty, large capacity rubber band to keep the lid and base together. These large bands can be found in office supply stores and are used to secure large files, etc. They're about 1/2" - 1" wide x 6" - 12" long.
Neither suggestion is very elegant but I think it would help. and if I've misunderstood, my apologies.
EDIT: Slow fingers did me in again.