You are looking for a bison container that only has room inside for the log.
As you walk to the cache you pass through a large open space with little more than some crops and perhaps the odd bird... it’s a field, have a bonus point if you correctly identified it as such.
... to get to the next cache in the series, turn right, still following the Hertfordshire way, and please watch out for the occasional car as you walk along Wall Hall Drive, singing either "Long and Winding Road", "Road to Nowhere" or "Thunder Road" whichever is your preference.