Mutual fund substitute for EDV to go with PRPFX?

[AdamA]

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.

[AdamA]

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. 

[MediumTex]

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.