You can also allocate 40% Series I Saving Bonds (perhaps the safest financial instruments in existence, yielding 2.83% right now), 40% Vanguard High Yield Tax Exempt (a good proxy for Vanguard's corporate bond index, except issued by municipalities), 20% long term treasury bond fund (VGLT/TLT/VLGSX). This will give you about the same yield, duration, and credit risk as a total bond index fund, with some extra possible diversification benefits. Importantly, it will also put 80% of your bonds in taxable accounts with low/no/delayed taxes, leaving the 20% which is most useful for rebalancing in tax advantaged space (assuming you have an option where you can fit 20% in). A useful way to both save and eat your cake.
But then, putting it all in total bond funds in tax shelters is about the same as above but simpler.
Edit: got wrong long term treasury fund tickers.