You may find that you need to increase the opcache.memory_consumption or some other setting for Opcache. To do so, you will need to create a custom configuration file with the settings you'd like to build Opcache with that DirectAdmin's CustomBuild will recognize. Then, copy the original opcache.ini file over to this custom/ directory.
cd /usr/local/directadmin/custombuild mkdir -p custom/opcache cp -p configure/opcache/opcache.ini custom/opcache/opcache.ini
Now, edit the custom/opcache.ini with your desired changes.
nano custom/opcache/opcache.ini ./build opcache
Opcache should now be configured with the settings desired and these changes will survive subsequent CustomBuild builds.
That's it! :)