In any Tesla coil the toroid (or top capacitance load) is a key component.
Safety warning: One thing to keep in mind is that it is not recommended to run your Tesla coil without a top load, although it will function (with a bit less output). The spark generated by the L2 may be fun to observe and tinker with, however it is dangerous by nature (in seen and unseen ways) and produces ozone.
There are several ways to approach toroids from a hobbyist perspective. Depending on available materials in your area, and budget, there are several avenues you can choose when seeking a toroid. There are generally two types of top loads- spherical, and actual toroidal shaped loads. Spherical loads are generally best for wireless power distribution, while the actual toroidal shaped loads are best for visible breakout arching.
Hollow stainless steel sphere
Wire toroid design
There are optimum ratios to achieve best performance per milliamp draw when it come to toroid design. However, available materials, cost, aesthetics are also a factors. Out of proportion top loads within certain tolerances generally do not have a negative impact on the overall performance of these Tesla coils.
A wire toroid design can be conjured if a metallic sphere or aluminum duct type is unavailable. A simple design for prototyping would consist of cardboard cut-outs, wrapped many times in thin gauge wire, or fewer times with a heavier gauge wire. Electrical tape can be applied along the windings to secure them in place. Once a sufficient density of wire is chosen, snip the end with enough left over to form a small tension spring to connect to the L2. Solder the two ends of the wire together to ensure that there is no voltage difference within the toroid. Remember, this is not an inductor( L3); its purpose in the circuit is a capacitor, not an inductor.
Metallic paint can also be applied for increased effect, however many types of paint are flammable, even after setting for more than 24 hours, so choice of paint is key (if desired).