Twin SUs move a lot of air, but there’s not a lot of room for a decent air filter. The factory pancake filters are a bit undersized, aftermarket oiled foam filters are often worse, and, while K&N filters have a bolt-on appeal, I don’t really trust them. Their marketing seems a bit snake-oily.
Plenty of other cars use disposable paper air filters, I’m sure I can find something common that will fit…
Oh look. It’s a filter from a 2.5 litre V6 Nissan Cefiro, also used on various other models. I wonder if it’ll fit…
That counts as fitting to me. Time to start designing the housing using CAD (Cardboard Aided Design).
And now translate it to steel:
The lid is a little more involved, since it has to clear all sorts of somewhat important things. No worries though…
A cold air intake from the wing, a lick of paint, a Leyland sticker of dubious authenticity, and we’re done!
(Ok, so that actually took 5 months from beginning to end. Here’s what it looked like for a while:
Not pictured here: building a new kitchen, landscaping the garden, various other things that had been put off for too long.)
A small obsession 
I am (once again) in a mix of awe and envy at your mad skillz (and shop). You’re right not to trust K&N’s claims; they’re total bulk wrap. See http://bit.ly/1q7aGXt and http://www.webcitation.org/5utsuCgAU .