For those that are interested, the design of the plenum IS NOT something you can just knock together and here's why.....

With the swedish style plenum the inlet (throttle) is at one end ahead of cylinder No.1 at right angles to it.

This put the point of inlet air closest to No.1 cylinder, no.2 is slightly further away, then no.3 with No.4 being the furthest away.

The problem this causes is that the runner lengths (throttle to inlet valve) are then all different. If left as is, no.1 would be supplying more air than No.4.

So to compensate, the plenum is tapered towards the rear.
By making this convergent this serves to increase the velocity of the air (thermodynamics), such that the airflow going to cylinder no.4 is speeded up in an attempt to get it to arrive at the inlet valve at the same time as No.1 (which has a shorter path).

You can see how the standard plenum supplies the inlet towards the middle of the plenum (actually closest to No.3) this therefore means No.3 get the best airflow over the others.

I started working out the maths and it got so in depth its far easier to make a test version and experiment with the maount of taper and fully bench test it until you can get the closest equal airflow across all 4. notice I say closest as that is what you'll get at best.

Considering the amount of money spent by the likes of Ferrari here's a picture showing the eventual (best comprimise) of the inlet plenum on the F430. You can see even they are not equal


So to be fair, you could try and make one yourself, but you need to give careful thought to the airflow and do various testing to ensure it is right. you could of course get lucky. Either way, I can see why people charge what they do for these since there is alot of work to produce the right end result.