Donald,
Also see
Battery Latch Open Error
where Mahstah has put a picture, and I added this comment :
"Can I add a bit to help prevent others spending the uncomfortable half hour I've just spent? I started with a piece of hard foam. It disappeared inside the machine. Oh dear. I tried making a wedge with a matchstick. Same thing. With the aid of sharp needles and tweezers I managed to get them both out again, and then started again, looking properly at what I was doing instead of guessing (duh...).
I thought that the switch mechanism was at the end of the left bay. Well, technically it is, but in practice it's the entire floor of the left bay which forms the mechanism - like a diving board, left end fixed, right end pressing the contact. So my solution was not to try again to wedge anything in the right-hand side of the bay, but to put the same tiny wad of hard sponge (cut from the rings you get holding down CDs in 25-packs) on the "diving board", then setting the catch on top of the wad and closing the catch. Result - the wad presses properly on the spring mech and holds it shut."
In fact my final solution was to glue the piece of foam to the hook on the cover so that it can't slip or get lost. It still plays up a bit, and occasionally takes a bit of wiggling to make it work, but when it does the results are worth it.
Good luck.