There are several methods of packing items in cache tubes that can all but alleviate all moisture problems.
The first is easy, packing the items in mylar or plastic bags. Vacuum packing or the like for many items works perfectly fine in this instance. This keeps any moisture or water that may get in the tube off the items inside.
Secondly, items that may rust require a different touch, first you need to put a protectant layer on the item itself, LEE LIQUID ALOX actually works great for this. A thin layer creates a somewhat hard ‘layer’ or crust on things that keeps water and air off items. However you will have to remove this before use. After Aloxing all surfaces, you again put the item in a mylar or plastic bag and seal it.
Once the tube is capped and the cement hardened on the ‘bottom’, one packs the tube carefully. A string to the bottom item or a cup/bucket to aid in removing the items, especially on the longer tubes.
Once the tube is packed, you take a canister of nitrogen and fill the tube with nitrogen to evacuate all the air possible. A lit match held at the tube top will extinguish when the nitrogen gets to it’s level. At that point, you cap the top, either with a screw cap or a normal cap end.
The screw cap sealed with silicone sealant, a little in the threads and a bead around the opening outside will keep most water and moisture out. The normal end cap can be sealed with cement or silicone depending on how long you intend the tube to be under and where you put it. Cement if underwater of course.
For those who may not think of it, take a coarse hacksaw blade and coat it with the ALOX when you’re doing other items, then package it in a ziplock bag and duct tape it to the outside of the tube to allow you access without tools later.
The screw caps can be removed with a couple of branches and a bit of paracord, making an impromptu spanner.
Dessicant packs work, as do oxygen absorbers, much depends on how long you are putting the items back.
For short term, a little oil and ziplocks will do fine. As the length of time increases, the needs and methods change.