The last thing you want is flexible materials. They will increase your internal volume.
If you want to permanently increase it, and don't have room for the usual brick / wood block ideas I suggest a 2-part mixable epoxy that dries hard.
You may also experiment less permanently with buck shot. Placed in bags it's relatively easy to arrange and move.
If you want to permanently increase it, and don't have room for the usual brick / wood block ideas I suggest a 2-part mixable epoxy that dries hard.
You may also experiment less permanently with buck shot. Placed in bags it's relatively easy to arrange and move.