Using this Cubical Quad Antenna Calculator, it appears that the length of your driven element targets 95MHz operation. As you surmise in your comment, wire insulation and PVC housing will reduce the velocity factor of the elements, which in turn will make the elements electrically longer and lower the antenna's operating frequency.
Tuning stubs are made from transmission lines; the word stub refers colloquially to a short section of transmission line. The tuning stub acts as a reactance in series with the antenna conductor: series capacitive reactance will shorten the electrical length, while inductive reactance will lengthen it. Applying stubs to the driven element and to the reflector will both affect driving-point impedance; applying a stub to the reflector will also change the antenna's directional characteristics.
The stub's reactance depends on its length, the characteristic impedance of the transmission line comprising the stub and whether the end of the stub is a short- or open-circuit. For short-circuited stubs:
$$Z_{sc} = jZ_0 tan(\beta l)$$
while for open-circuited stubs:
$$Z_{oc} = -jZ_0 tan(\beta l)$$
where $l$ is the length of the stub and $\beta=\frac{2\pi}{\lambda}$. Since $\lambda$ is the wavelength in the transmission line, the line's velocity factor must be figured into the calculation. Assuming you want the shortest practical stub, $\beta l$ should be kept to less than $\frac{\pi}{2}$, the length at which the sign of the reactance changes, so $l$ should be less than $\frac{\lambda}{4}$.
Since you are operating in the vicinity of 100MHz and using only 10 watts of power, it will probably be most convenient to make your stub from an inexpensive RG-58 or RG-59 type of coax rather than making your own transmission line. 50-$\Omega$ RG-58 coax is popular in amateur radio, while 75-$\Omega$ RG-59 coax is commonly used in CATV installations. Using RG-58/U as an example,
$$\lambda = \frac{v c}{f} = \frac{0.66 \times 3\times10^8 \left.\mathrm{m}\middle/\mathrm{s}\right.}{95\times10^6\mathrm{Hz}}= 2.08\mathrm{m}$$
and $\frac{\lambda}{4}\approx0.5\mathrm{m}$
Changing the length of an open-circuited line is simply a matter of cutting the line to different lengths. The length of a short-circuited line can be trialed by inserting a pin in the coaxial cable to short the braid to the center conductor.
