Ferrous meaning Fe or Iron & steel
Non-ferrous – Aluminum, Brass
The metal detectors can distinguish between the two types
Sheet metal may be enough of an RF shield to mask whatevers underneath, but it may be useful to have several layers of cheap, flat metal to confuse the issue
Pie Tins might be a good choice – Everybody likes pie, so save the tins for a “Reverse Treasure Hunt”
I would bury several dozen of those high powered super magnets along with a couple rolls of pennies to mimic bullets, maybe toss in some fishing weights. And just to get more imaginative do the pipe bit but just use some 4’ ABS about a foot long with caps, toss all the above in and glue it up.
Bury these as another poster commented in obvious places, make sure they are upright in case a portable ground radar is used.
To shield the real deal I would make a shield of a rubber mud flap that could have something dense like spraying it with glue and throwing a layer of lead shot on it, or get some lead foil. Having it under a tree is an excellent idea.
Course anything said here and now is being noted so be adaptable.
Good advice! I have lots of pie tins.