My favourite "plain" cross-zip is the Buco JH-01:
The Langlitz cascade is pretty plain too:
Like has been said before, an Aero Bootlegger, with or without D pocket are pretty easy to wear:
The Vanson CHP2 and Chopper are also pretty plain IMO:
As are the variations of the PJ-27
Or a Schott 620/621:
The Langlitz cascade is pretty plain too:
Like has been said before, an Aero Bootlegger, with or without D pocket are pretty easy to wear:
The Vanson CHP2 and Chopper are also pretty plain IMO:
As are the variations of the PJ-27
Or a Schott 620/621: