I know that this probably wouldn't make any difference, but have you tried a different transponder in the key case, it shouldn't make any difference, but it would eliminate an issue with the transponder itself.
Also I'm not sure of the exact way these work, but would it be possible to put some sort of conductive material (e.g. a sliver of foil) in the key case, to effectively make the distance from the actual key to the transponder closer, if that makes sense?
I'm keen to find the outcome, because I found these on Ebay the day before you posted this thread and I was very interested because I have a key that is held together with masking tape, and I miss the flip type key I used to have with my last car.