I found this research article where they did tested the antimicrobial activity of some plant oils using Agar Well Diffusion ( Esmael, A., Hassan, M. G., Amer, M. M., Abdelrahman, S., Hamed, A. M., Abd-raboh Hagar A., & Foda, M. F. (2019). Antimicrobial activity of certain natural-based plant oils against the antibiotic-resistant Acne bacteria. Saudi Journal of Biological Sciences. doi:10.1016/j.sjbs.2019.11.006).
I presume they use parafilm to seal the Petri dish; but how would you prevent the oils from evaporating from the well and settling onto the agar surface?