many thanks for sharing this interesting technical question with the RG community. Zinc chloride has been found to be an excellent reagent for preparing solutions of well-defined density for measuring the density of microplastics. Such solutions can be prepared up to a final density of 1.8 g/cm3. Thus your desired density of 1.5 g/cm3 is easily possible in this system. For a detailed protocol how to make these reference solutions of ZnCl2 in water, please have a look at the following very useful article:
A straightforward method for measuring the range of apparent density of microplastics