There are a number of ways you can make these.
1. You can make 2 moulds (upper and lower) and bond the 2 carbon parts together to make the 1 part.
2. Using a 2 part split mould (upper and lower). You then need to compress from the inside of the 2 parts....again theres a couple of ways of doing this
i. Use an internal bladder. Inflate it to increase the pressure and push against the moulds
ii. depending on complexity you could use a solid buck inside. The 2 upper and lower then compress the carbon against it but you risk it crimping on the join.
I have heard that silicone expands when heated but i have no experiance of that myself.

Im trying to make something similar myself for a turbo link pipe for my bike. Its basically a long U shape. Ill be using 2 part mould and an internal bladder. The carbon will be layed up in the 2 halves, clamped together and bladder inflated. The join will not be too visible.

hi im fairly new to making carbon fibre parts and i have been wandering for a while now, how are carbon fibre intake ducts made, i have looked in quite a few places and on this forum but there is no real good answer