You'll have more luck with the second tier top prospects. Kyle Zimmer, Taijuan Walker (who could be considered first tier), Jose Berrios, Richie Shaffer, and Bruce Maxwell all come to mind.
Austin Meadows is also a great signer. Clint Frazier signed before he got drafted, but I'm unsure about how he goes about his fan mail now.
Currently Buying/Trading For - Daniel Robertson