Why not shrink the Memory Box down to hide Mari?
Setting aside unknowns like the Box's durability to avoid it breaking during this process - assuming it were possible they could've shrank it down over and over to essentially crush her into a tiny compressed pill of unrecognizable remains then buried it. Like, the size of a Tylenol. Practically impossible to find out in the woods due to small size, presumably airtight and there's canonically no decomp so there's no scent for animals to dig up, seems foolproof.
Even if it only got down to the pencil-case size its introduced as, that's still way more discreet than the full on casket size they leave it as.
Of course they're 9 year olds and probably wouldn't think to do this, also there'd obviously be no plot if it happened, just a random thought on my 10th+ rewatch.
Love ya!