Gyroscopic Investing

VUSTX would probably work okay for you.  It's not as volatile as EDV, but it will give reasonable exposure to long term government bonds.  It does that Vanguard 80/20 thing (80% Treasuries, 20% other stuff), but it's only 10% of your portfolio and would probably still give you enough diversity to improve your yield a bit. 

FLBIX (Fidelity) might work even better.  It's essentially the same fund, but, as of yet, Fidelity keeps it 100% Treasuries and has not done weird stuff with 20% of its holdings.  The minimum is $10000. VUSTX is $3000.

TennPaGa wrote: Do you all have a sense of what would be the "right" breakdown between PRPFX and FLBIX? 

I don't know for sure, but I think I asked a similar question in the context of TLT/PRPFX for a taxable account and the consensus was to stick to 10/90.

Keep in mind that you could probably do fine using 100% PRPFX, so the bonds are pretty much icing on the cake.  My gut tells me that 90/10 is probably still fine.

Hopefully, someone more knowledgable than me will give you some input. 

One other thing I know is that American Century offers some zero coupon mutual funds.  There is one called BTTRX that looks to have been as volatile as EDV (as far back as I can check on google finance).  The thing that I don't really understand is that it is set to mature and liquidate in 2025.  You could look into to it, but I don't know if it's worth the extra weirdness.  I'll bet FLBIX works out fine. 

I am pretty sure there is no right answer, other than to try the various approaches out for yourself and see how it feels to live with it for a while with your own money.

I also suspect that any of these approaches are going to provide similar returns.  The question is which one will provide you with the most peace of mind.