Try this

packages/py-spinnfrontendcommon/package.py

@@ -5,6 +5,7 @@
 
 from spack.package import *
 from spack.fetch_strategy import URLFetchStrategy
+from spack.store import find
 
 _JAR_URL = "https://github.com/SpiNNakerManchester/JavaSpiNNaker/releases/download/7.0.0/spinnaker-exe.jar"
 _JAR_SHA256 = "2d909c7fb3aa15886acf26febb1bd48e25db0c347a231944aa6a5f86107bb55b"
@@ -28,3 +29,11 @@ class PySpinnfrontendcommon(PythonPackage):
     depends_on("py-ebrains-drive@0.5.1:", type=("build", "run"))
 
     depends_on("java@11:")
+
+    def install(self, spec, prefix):
+        super(PySpinnfrontendcommon, self).install(spec, prefix)
+        # Work out the python version installed, so we know where to put
+        # the java code!
+        python_spec = find("python@3.7:")
+        python_lib = f"python{python_spec.version}"
+        install_tree("JavaSpiNNaker", prefix.lib.join(python_lib).site_packages)