"Pumping" effect on track at quarter-note intervals

I have a bass instrument track that has a bass note every eighth-note.  
 
I also have a synth track with some long notes that I want to create a "pumping" effect at quarter-note intervals.
 
What's the best way to do this? 
 
I already tried a few things but didn't have success:
1) Tried putting a sonitus compressor on the synth track and sidechaining in the bass track, but that resulted in pumping every eighth-note.
2) Tried automating the volume with a sine wave, but I find that tool very hard to create a curve with consistent amplitude across the entire track.  Like I couldn't figure out vertically where to start drawing the sine wave so that the amplitude modulates the right amount and at the right level consistently.
 
I know I can copy the bass track and create a new instance of the synth, change it to quarter notes and then use that as a sidechain input for the synth, but that solution isn't very efficient. 

The easiest way I've found to do that is to use a tempo-synced tremolo plugin. Meldaproduction has a free one that works great and lets you modify the modulation waveform so you can make the effect as subtle or abrupt as you like.