Do I need a new port for my 100mm macro lens?

I'm about to buy a new housing and am building up a wish list based on my lens collection. I can either buy a port that has been designed for my Canon 100mm IS L macro lens, or use the same port as for my 60mm with spacers which would result in a 7mm gap between the end of the lens and the glass of the dome, but saving £300 (almost US$500).

Any opinions please, wise Canon owners?

Shouldn't make any difference at all and that leaves you space for a smallish diopter. The 100mm can't focus that closely, unlike the 50mm macro.

The only downside is an extra failure point.

I constantly use an extension ring for my 100mm in my Seacam, varying between lengths depending on extension tube or teleconverter or both!

Thanks Drew, I appreciate your advice, and am saving myself a few quid.