Bump . . . . . . this thread might get you thinking about some things. As might this: Bury a gun and ammo for 15 years by Charles Wood Issue #115
Another tip on using the PVC pipe and burying in a posthole fashion.....The eyebolt is an excellent idea. To go a step further in making retrieval easier, sleeve the capsul being buried with a larger diameter pipe, open on the bottom, screw cap on top. It could even be a second capsule, sealed on the bottom to further protect your cache, with a screw cap on top. Doing this would have the huge benefit of allowing the retriever to unscrew a cap and slide the capsule out of the sleeve, as opposed to digging the entire length of the capsule out of the ground. A 6-10" diameter PVC cap will have +/- a 1/4" lip around the pipe. This, after the ground settles around it, will not allow the capsule to freely slip up out of the dirt or gravel, no matter how well it is planned out. When you need it most, you'll spend way more time digging it out than you planned. With the sleeve, once you access the end of the outer capsule, you simply unscrew the cap and lift out your bounty.
One word of caution when caching; Never bring a smartphone along with you when scouting possible cache sites or placing supplies on a site. Even if the phone is turned off. (Anyone notice you can't remove the battery on the newer phones anymore ?) Use a standalone GPS if you need to be hi-tech or just use a topo map and triangulation if you can't remember things.