Florist Foam ... or use dremel tool on a real brick. Perhaps a small real Lock Lock could be used instead of a film canister. Would be watertight and more fun to be able to leave signature items and travelbugs.
I found one very similar to this while on a recent geocache hunt over 1000 miles away from home.
Embellish Shorts | Crafts Beautiful project | Dremel
Hummingbird Stone Home decor by sjengraving on Etsy, $25.00 use with polymer clay to stamp the pattern
Sea glass and beach stone for you and your luva!!
BEACH STONE BUTTONS... 5 beach stone button tags-earth history-pebbles grey rock beige natural findings-sewing notion
Earthy Porcelain Pendant with Queen Annes Lace
Klippies maak ook ongelooflike hangertjies! pebbles (originally spotted by @Caroleckb229 )
Piedras talladas con dremel
Piedra tallada con Dremel - which possibly translates to carving into a stone with a Dremel tool? Neat idea!