Those bags come in rolls,,,you can make them any length you wish...
Once you get it bagged you can store it in places like the pipe...
This Llama was a "gift" from a "client". It actually shoots well, but is expendable. If it fell in a well, it would not be a great loss. When vac-packed, they store forever with no rust or corrsion, this piece has been in this bag for about 7 years. But when dropped in a coffee can and set out of sight in an out building, or other strategic place, it will keep forever. It may accumulate dust and mud daubers, but it will always be servicable once you open the package.
The pipe comes in all different sizes, in 20 foot joints, but you can get shorter pieces at a plumbing shop. Cap the other end. One of these was just dug up in Oregon, where it had been hidden in the forest for 10 years, and the rifle inside was rust free. Ammo bags real nice, and stores in a safe easily. Think outside the box, dont put all your STUFF in the house.
PVC is a popular means of caching stuff because it works. The biggest problem most folks have is forgetting precisely where they cached their stuff. Make sure you have good landmarks that won't move and a means to locate your cache very, very accurately unless you enjoy digging.
Storing PVC pipes under docks is another common method.
The idea of not keeping all your stuff in one place is smart. If you have one or more bug-out locations, each location should be stocked. Don't think you're going to lug your provisions from home to your BOL.
Most people are going to prep and stash in familiar territory,,,most people wont be caching in uncharted territory, so it isnt going to be an issue unless the person were to die, in which situation, some other family member should be in the know anyway.
I use GPS in my business, and I find it to be very accurate, I can drive a pin and then later return to within a couple feet of it, close enough to find it again with the metal detector, so, just common knowledge and as you say grid map, I could have gotten to within detection distance anyway. Ha. Then send the map home with your son or daughter for safe keeping, lest it fall into enemy hands..........ha. Some people just need a check up from the neck up.