Making particular shape  will be difficult for all materials .Even though, better to make round shaped pellets ..then it will cover maximum area... and use it as a dielectric material in between two parallel conducting plate... The capacitance is proportional to the area of cross section of the dielectric...So round shape will give maximum area compared to other shape....

To measure capacitance Metal insulator metal is a useful structure. You have to make sure that any barrier capacitance is not coming to the measured capacitance due to metallic contact. To check it you vary the thickness of your insulator. If your measured capacitance decreases linearly with thickness, you can be sure that barrier capacitance is absent. You have to eliminate all other stray capacitances by making three terminal measurements and using shielded cable connections. To avoid moisture and for electromagnetic shielding, you can mount your sample in a metallic sample holder and create vacuum inside. Keeping all theses prequations you can measure capacitance accurately.

Use MIM structures. Just evaporate Al, deposit your dielectric and finish the device (essentially a parallel plate capacitor) evaporating second film of Al (e.g. dots or strips). Use a capacitance meter at a given frequency (e.g. 800 Hz, etc.) to measure the capacitance of your capacitors. Alternatively, if you fabricated bottom gate, bottom contact OFETs you can measure the capacitance directly between gate and source or gate and drain.