You need a BBU bracket accessory and a dummy tray. the BBU will be installed on the dummy tray so you can use all the 4 3.5" HDD trays.
The parts you need with SMC part number as below
BBU bracket BTR-0018L-0000-LSI and
1. floppy dummy tray MCP-220-83601-0B, and it occupies the space for USB/COM tray.
or
2. MCP-290-00036-0B, and it occupies the space for DVD tray.