No u can't. I just built 1. Spent:

£4 - 5" to 4" reducer
£8 - fencing support the carbon
£6 - sheet of cooking hood carbon filter


Total = £18

Looks the shit and works really well. For the base of the carbon filter I cut the base of a 50 cd container to leave a tube, then glued this to an old paint lid and stuck the fence around that, and the 5" to 4" reducer on the other end.