I already had the compact macro port base for my 60mm so instead of getting the monster NX Custom Macro port, I went with a longer Compact Macro Port (M?) and picked up a zoom gear from Athena. Reduces the # of ports I need to carry around when I'm bringing both a 60mm and 100mm macro lens. Works fine, but given how negative this housing is, it might be better to get the bigger diameter ports for additional buoyancy.
They have also come out with a sleaker looking DX port. S&S now has their own focus gear that will fit in either the DX port or the Custom Macro Port.
Accordingto the system chart below, I guess you could reuse your SX Extension with the NX Custom Macro Port.
http://www.seaandsea.jp/products/img/canon_digital1.gif
** IMHO, I strongly suggest getting a focus ring with this lens as the AF seems slower/ hunts more on the 100mm macro USM than the EFS 60mm macro and I find I need to manually fine tune the focus once in awhile.