I just done same to my Fender Modern Player Jazz 4. But this time with wood filler. I have done numerous fretlesses and this was first time with filler. I think that proper way to do this is with wood veneers, but if you are hurry, like I was, filler is way faster. I used Plastic Padding Chemical Wood. It dries in 5 minutes,so it is pretty fast to do whole thing in few hours. I have been put J-Retro to it earlier and it was nice with fretted version but now with fretless version of same bass it is unbeliviable good. This half parametric mids you can amount some serious life to your sound and findin' that right mwah J-Retro is right tool for the job.