I use the staywell battery magnetic catflap. This uses coded keys, just like the infra red ones. But if you wanted you can code in a RED key and then ANY red key will work. This is what I do with mine having 5 cats as the catflap will only program in 2 different keys...
Anyway when you set it up, you hold it about 2 foor back, pointing in a certain direction and then move it into the center of the catflap until you here a bleep (I think, been a while since I set mine up) then it will ask for another key. Dont worry if its just one cat, program in the other key anyway to shut it up hehe. They usually set pretty far back when you are programming them in, so I would think that if it is back the back of the cets neck it shouldnt matter. And by the time a cat is banging at it trying to get in, it will have either moved or the neck is far enough in to trigger it. Also with the weight of them (not heavy) if they cat has been walking/running around outside the chances are it will be hanging in front of the neck anyway.
The only downside to the catflap are-
-If they loose their collar they cant get in, a 2 pack of replacement keys in the shop is £7.50, but ive managed to pick some up as spares in ebay for £5.
-When the battery goes (it makes a bleeping noise to warn you its running out first which lasts 2-3 days) you have to reprogram in the keys, so you need the cat around
The catflap itself takes 4 (I think) AA batteries. Buy decent ones like duracel plus or something, not cheap ones they just dont last. Other than that I have had it for about 3 years now with no problems. If your cat looses her collar and you have to wait to buy a spare key what I did was just tape the little red thing that stops it opening without a key, but remember then ANYTHING can get in.
I wouldnt reccomend the round magnet ones, they are pretty powerful and I will never forget the day Shelly (Died before I became a memeber here) came home with a tin can attached to the magnet
Do you have many strays/cats likely to come in while you are away? If not I would prob tape down the catch, then it doesnt matter if she looses her collar or not, she can still get in.