On-chip electron spin resonance ESR of magnetic molecules requires the ability to precisely position nanosized samples in antinodes of the electromagnetic field for maximal magnetic interaction. A method is developed to entrap microcrystals containing spins in a well defined location on a substrate's surface. Traditional cavity ESR measurements are then performed on a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr 5+ spins are entrapped as well and measured while approaching the lower limit of the ESR sensitivity. This method suggests the feasibility of on-chip ESR measurements at dilution refrigerator temperatures by enabling the positioning of samples atop an on-chip superconducting cavity.