The air intake seems very effective to be fair, and not including the filter came in at under £40.
Originally I was going to make the lid out of carbon fibre, and now I know it's effective and looks okay, I probably will do at some point, but the carbon sheet in that size was coming in at £80.
It's made using 1mm sheet steel from b&q £25 and a length of 2mm aluminium angle £8. I removed the rubber weather strips from the left side and top of the engine bay then laid the length of aluminium angle from the back of the bulkhead to the front slam panel to get the bonnet shut line.
Next up made a cardboard template from the original rubber airbox mounting points to just below the aluminium angle (allowing space for the lid and bonnet to shut. Also at this point marked the hole for the std turbo to MAF pipe, this pipe was handy as it's got some flexibility for positioning. I used a 3 1/4" hole saw and cut and smoothed the edges out.
Next up transfer all this over to the sheet steel and cut out and tidy up. Take your aluminium angle and cut two pieces the same length as the top and bottom of the plate. Drill and fix both of these flush with the top and bottom of the plate. I used M6 x 12mm bolts, but pop rivets would be neater here.
Mark up the holes on the bottom plate that line up with the STD airbox holes. I used two M8 x 20mm stainless bolts with a nut on the back to make pegs to fit into the holes. This plate will need covering in some sort of sticky back heat shield. I got some off ebay from a motorbike supplier for £10 that's rated to 600c. I also used some sticky back foam across the top that I had laying about.
Next up made the template for the lid from cardboard, then transferred it over to the steel and cut it out. 1mm thickness is ideal here as it fits nicely under the lip on the wing and the bulkhead but also allows the STD rubber weather strips to fit back in creating a nice tight seal and also keeps it firmly in place. I chose to wrap mine, but obviously painting is also fine.
There is also a nice cold air feed into the bottom of the box from the front bumper.
A few pics...