We went to elaborate cat jailing precautions in Cape Town as escape would have been disastrous. Our back door had a security gate fitted (outside; same size of the door) with a plastic mesh over it to prevent escapes. Looks horrible but worked. The front doors both there and before that in London were unsecured as we couldn't see any way to do it (and OH drew the line!) so I used to come in stamping my feet and making loud WAAAAAH noises and without any shopping bags etc (all put down outside beforehand) , locate and shut up cats to bring shopping in etc. Guaranteed to cause maximum curtain twitching from neighbours btw
. Effective though you always have to be on alert to retrieve a feline that shoots past - if they know they're not allowed out, *generally* they shoot out and then hesitate in the unfamiliar surroundings, enabling you to recapture the fugitive! (hopefully). I do know how scary a prospect it is though.
Otherwise think your Mum is on the right track but that is expensive & ugly - you'd have to build a frame fixed to the wall, maybe even with a gate in it to give you the 'double door effect', then have it wired up or a blind put in. Do you have a front window as well as a door? If you come home with someone, maybe (this occasionally worked) they could make faces/noises to attract the cats while you go inside, then capture the wrigglers until everyone is in safely.
good luck with it all! We spent a fortune in catproofing but worth every penny for peace of mind.