diff --git a/cantrip_tests.mk b/cantrip_tests.mk
index 92ee54e..bc7bb0b 100644
--- a/cantrip_tests.mk
+++ b/cantrip_tests.mk
@@ -54,7 +54,7 @@
 
 ## Runs cargo unit tests for the MailboxDriver
 cargo_test_mailbox_driver: | cantrip-gen-headers
-	cd $(CANTRIP_COMPONENTS)/MailboxDriver/tests && $(CARGO_TEST)
+	cd $(CANTRIP_COMPONENTS)/SecurityCoordinator/tests && $(CARGO_TEST)
 
 ## Runs cargo unit tests for the opentitan-timer support
 cargo_test_opentitan_timer: | cantrip-gen-headers
diff --git a/platforms/nexus/sim.mk b/platforms/nexus/sim.mk
index 38a7c74..a472161 100644
--- a/platforms/nexus/sim.mk
+++ b/platforms/nexus/sim.mk
@@ -49,12 +49,9 @@
 	ln -sf $(CANTRIP_ROOTSERVER_RELEASE) $(TMP_RELEASE)/capdl-loader
 	tar -C $(TMP_RELEASE) -cvhf $@ matcha-tock-bundle.bin kernel capdl-loader
 
-# NB: Package the builtins bundle so it can be written to a carve out in
-#     in SMC memory; this is temporary until the SEC supports returning
-#     the builtins from flash.
+## Packages the builtins in the tarball for writing to SPI flash.
 ext_flash_release: $(EXT_FLASH_RELEASE) $(EXT_BUILTINS_RELEASE) | $(TMP_RELEASE)
-	ln -sf $(EXT_BUILTINS_RELEASE) $(TMP_RELEASE)/cantrip-builtins
-	tar -C $(TMP_RELEASE) -rvhf $(EXT_FLASH_RELEASE) cantrip-builtins
+	tar -C $(CANTRIP_OUT_RELEASE)/builtins -rvhf ${EXT_FLASH_RELEASE} $$(${CPIO} -t < ${EXT_BUILTINS_RELEASE})
 
 # Dredge the platform configuration for the physical address where the
 # cpio archive is expected.