You can get the filter holder and adapter rings here:
http://www.bhphotovideo.com/bnh/cont...rch&Q=&ci=3848
http://www.bhphotovideo.com/bnh/cont...rch&Q=&ci=3847
The start-up cost may be a little more, but the ability to change the elevation of the ND split and adapt the filter to any thread size is just great.