Any ideas for feeding of outdoor cat while on vacation?

Here's the situation: A feral cat had kittens in my backyard 2 years
ago. I fed her while she was nursing. Handled the kittens daily and
found homes for them. Had the mother checked by vet and spayed. Felt
bad for her after taking away her babies, so continued to feed her.
Clever cat found way into my detached garage and for 2 years all was
well as I fed her in the garage and she had a warm place to sleep.
The inevitable finally happened and several feral cats took over the
garage. I had to cat-proof the building and am now feeding her on the
back deck - but if I step inside for the least little minute there's
one or another cat eating her food. "My" cat backs off immediately -
she's a real scaredy cat, but maybe having only one eye puts her at a
disadvantage. (BTW, these other cats do not appear to be starving -
none are thin and I know of at least 2 neighbors who leave food out
(their yards are FULL of cats, which is something I'm trying to
avoid).) Anyway, I'm at a loss about what to do when I go on
vacation. Previously, I had a neighbor fill the food dish in the
garage, but this is no longer an option. Nor can I ask the neighbor
to guard the food dish while she eats, b/c she will not go anywhere
near another human. She's become dependent on me and I hate to think
of her waiting day after day for me to feed her - I don't see her
invading the neighbors' colonies for food. Does anyone have any
ideas? All I can come up with is an electronic door/collar installed
in the garage, but I rather doubt she'd leave the collar on and I'm
concerned about the safety of an unsupervised outdoor cat with a
collar. Thanks so much for your time - any help would be greatly
appreciated.