Maybe if there was some way to assign three judges top each Senate class. They could have 18-year terms. Maybe something like this:
2000 (Class 1)- Justice 1
2002 (C2)- J2
2004 (C3)- J3
2006 (C1)- J4
2008 (C2)- J5
2010 (C3)- J6
2012 (C1)- J7
2014 (C2)- J8
2016 (C3)- J9
Justice 1's seat would be up for replacement/reelection in 2018.
I like the idea of a single 18-year term for each justice (along with permanently setting the Court at nine justices), but I'm not sure what your idea of putting justices in classes accomplishes. I do think 18 years is plenty of time, not to mention that it'd at least somewhat reduce the impetus to choose particularly young nominees. It would also allow a one-term President two nominees and a two-term President four.
(I'd also alter your proposal so that justices were up in odd years instead of even years. I think it'd be best to not have them up in election years.)
And to those that argue for electing judges, I cannot think of anything worse that would utterly destroy the concept of an independent judiciary. An independent judiciary that upholds constitutional principles protecting minority rights is extremely important. Otherwise, there's no point in even having a written Constitution.