Mortgage backed securities.

There is the Vanguard GNMA Fund Investor Shares (mutual fund), symbol VFIIX and the Vanguard Mortgage-Backed Securities ETF, symbol VMBS . You could also buy a mortgage REIT.

If you consider a mutual fund remember that unlike the actual MBO a mutual fund has no maturity date so there is no point in time at which you will recover your original investment. The value of the fund shares will go down as interest rates go up. Considering how low interest rates are today there is a very high probability that rates will go up over the next few years causing a capital loss for fund investors.