I used 4 neodymium discs of 60x5mm enclosed in an iron structure, so that I get about 0.42T. I measure the NMR of the water, so I get my 18.54MHz.
I also initially tried the earth magnetic field NMR, inspired on Terranova device, but the signal is so weak that there is a lot of noise. As you said, the signal is so slow that you don't need any Red Pitaya, just op amps and normal ADCs from any microcontroller are enough for that purpose.
Good luck!
