Take a look at the Fotodiox Wonderpana 66 system. They make a threaded multi-coated CPL and ND filters that screw into their adapter for the D FA 15-30. This adapter also can hold the square/rectangular ND filters, and you can stack one of the square filters while using the CPL or round ND. There are a couple combinations you can buy things in, so price the pieces carefully. Look at the kit that has the round mount and square mount and lens cap, and then add your NDs and CPL.
Mounting for circular and 6.6" square:
FotodioX WonderPana FreeArc Core Unit WNDPNFA66CORE-KIT-TMR1530
and this gives a combination of kits:
| B&H Photo Video
note that they make a CPL that is not multi-coated and is bundled with the circular mount, and from what I can tell based on reading reviews, the multi-coated one is much better regarding flare. I went with the mc one, and so far pretty happy with it.