Have you spoken with Peter Mulberry at Mulfab?
http://www.mulfab.co.uk/
Have no idea on price, but am certain he could do this for you if you had CAD dwgs. Would be interested to know how much this type of combined inlet/manifold combined would be...and have often thought there's a market for this.
As note, looking at the pic I would have thought pre-fab'd extruded inlets would be easier as they're probably available off the shelf in a semi-standard size rather multiple length welds.
Do you have any more pics/dimensions of this unit?
Andrew