Can you use 'Rubber,' Specifically Liquid Electrical Tape Between The Plates of a DIY Capacitor
Sure; the capacity of a plate capacitor (Wikipedia is your friend!) proportional to three things:

The area of the opposing plates (the larger, the better)
the inverse of their distance (the closer, the better)
the dielectric constant of the dielectric (the higher, the better)

Your isolating  will probably have dielectric constant higher than that of air.
What it also might have might be a higher or lower isolation strength (i.e. at which voltage there will be arcing through it).
Also, there might be higher losses than in an air-filled capacitor when using the capacitor at high frequencies.