You can practice contact moulding by using something like a plastic food container of some sort as a mould, and then using GRP to make an exact replica of the inside of that. You need to try to avoid any air pockets, and use a brush to try and avoid part being resin rich.
There is no real point in contact moulding carbon, as this will mean a higher resin content than the ideal, which will mean reduced strength. Not an issue if parts are cosmetic only, but for good strength tou will need to use vacuum.
Main problem with carbon though is getting a good cosmetic finish, and this is an issue even if you are making parts using pre-preg in an autoclave.
Its also worth remembering that if you want to make copies of original bike parts in composite, then in many cases you can use the original part to take a mould from, which means there is no need to mess around making plugs.