It's not a bad idea to start with Sawtooth Software-generated designs, and then to tweak them a little bit in Excel to enforce certain properties you need for your study. Rather than looking for entire design versions that comply with your needs (and throwing away entire versions that fail), you might just write some IF-Then code in Excel that identifies offending tasks and tweaks them a bit to satisfy your needs. Typical tweaks involve randomly selecting attributes to change to another level. Or, identifying an offending level and changing it to another level (typically with some sort of random selection).
BUT, after doing any tweaking of designs (and even before!), make sure to test the final design using the Advanced Test option that produces simulated respondent data, a logit report, and resulting standard errors for the parameters of interest.