Hi ironjaycuttler!
Take DjVu plugin for IE here:
http://hamradio.online.ru/djvu_dwld.htm
Look at the page 26 (I mean number of page in a printed book), there you will find beginning of mixers description.
Page 26 - diod balanced mixer (Fosc = Frx). Left wire is from signal, right to amplifier, bottom wire to oscillator. You need not to put C5 capacitor, becouse you will need frequency band down to DC.
Page 27 - left from signal, right from osc, bottom to amplifier. Fosc=Frx .
Page 28 - left from signal, right to amplifier, bottom from osc. You will need not to put C4. Fosc=Frx/2 .
Page 29 - Look at circuit on top. Left from signal, right to amplifier, bottom from osc. Fosc=Frx/2.
Complete DC receiver you can find on page 56.
Use silicon diods with low capacitance. Oscillator voltage on mixer input need to be near 0.7 - 0.8 V.
All these mixers have dynamic range 70 to 80 db/mkV. For more big signals you will have parasitic AM. I think it is not very essential for you, becouse you will have no additional signals at input.
If your oscillator will make square shape signal then you will have more receiving of harmonic frequency 2*(Fs-Fosc). Maybe it is not so important for you. Anyway you can make LC filter to clean oscillator shape.
Heh, I have never used DC receiver to measure liquid flow by ultrasound, so maybe you will need some additional view on this problem.
Best Wishes! klug.