The Lee holder will take any brand of 105mm polarizer...the Lee tends to vignette quite a bit at wider focal lengths...best option would be a slim 105, but they can get quite pricey.
Lee also has a 4x4 square glass circular polarizer that slides into the holder...you can simply rotate the holder to get the desired amount of polarization. One bit of caution...the square polarizer will only "work" as long as you keep the engraved markings on the filter facing the subject...if they're facing you, there's no polarization effect. One other bit of caution...since the filter slides into the holder, you can use it in conjunction with graduated filters. However, in this case, to get proper polarization you'd need to remove the polarizer from the holder, rotate it 90 degrees, and slide back in, which could limit the amount of polarization you'd get.
I use this polarizer occasionally, however I do keep multiple round CPL's in various filter sizes...not a fan of step up rings.