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So looked & searched etc.etc. & seen the various evo/renegade discussions, but although many have done both, there seemed to be some reluctance to depend on either the total sealing capabilities of one type or the air flow charictaristics / warm air with socks/filters over the inlets.
Not sure if this type of mod has been done before but this is what I have done.
So, looked and have sorted a larger sealed airbox (approx 10%-12% increase) maybe not the biggest volume increase, but gives the security of being sealed so no dirt enters, retains the open aspect around the inlets, so no issues with socks/filters directly over to disturb the airflow & you can still utilise the 'probable' ram-air effect of the standard set-up.

Will try to post pics, but hopefully should be able to follow this:
Go get a spare lower half of a spare airbox.
Lay it face (flat) side down on the workbench, use a spacer and mark a line about 12-15mm high all the way around (I left the foam under the tank, but if you remove this may make abit more room for the 15mm)
Make sure your line is level & use small hacksaw to cut along the line, file to make smoonth & level.
You should now have a 15mm tall 'ring' made from the top of the lower half of the spare airbox.
Knock out the threaded brass inserts which you usually screw the lid onto.
Take some self adhesive 12mm wide draft excluder & stick this to the lower cut edge to create a gasket on the bottom of the cut ring/spacer.
Now place this spacer into position on top of your existing bottom half of the airbox on the bike.
Take the top of the airbox & place into position onto the spacer.
SO at this point you are looking at a std. airbox with a spacer about 15mm in between, giving you the increase in volume of the box overall.
The rest is basically the same as the tank version, foam onto a standard filter to take-up the gap & use longer bolts to join it all together.

Tank now sits about 10mm higher, but with new bolts, it still all fits just below the top yoke.
Hope you can follow this, will try with pics but I think you should get the idea.
Not run it on the road yet, but can't see any reason why it should not work in principle.
Heres hopin.
O.G.

Nice work

Verrrrrryyyy interesting. I have had thoughts along similar lines, except I've bought a 10mm sheet of Tufnol which I intend to use to make a spacer between top and bottom halves of the airbox, in much the same way as old git made his, and then put 5 mm self-adhesive foam on top of that. This should increase the airbox volume by approx 15mm x whatever the cross-sectional area of the airbox is.

I got the idea from another forum on another planet far far away... (OK, you get the drift), where the guy managed to track down an air filter of the same diameter as ours, but about 15mm higher, and of a high-flow type. It was originally fitted to a Saab 2.3 turbo, so should flow enough air.... Unfortunately the link expired a year or so back and so the original post has been lost.

I haven't got round to fitting it all together yet (no surprises there, then), as it's almost certainly going to need the tank raising by a few mm. But I've never been happy with the idea of having a semi-open airbox, or one whose sealing properties rely upon vaseline and a strip of car boot sealing rubber, so this seemed the best way to me.

Good work, old git. Perhaps this might be the jolt that gets me off my arse and doing something with my 'project'...

I like the idea - very clever.

You say that the tank now sits 10mm higher - what did you have to change on the tank mounting brackets to get it to sit properly?

(I'm assuming you have to raise the bracket where the tank hinges backwards and have spacers at the front, right?)

Tank raising:
All I have done is remove the rubber gromits from the front front tank mount plate, complete withe the metal spacers.
Now put them under the front bracket to act as a spacer between the tank bracket at the front & the frame to 'fill in' the appprox 10mm gap.
Use longer botls & a washer up top & hey presto tank is held.
(takes alot of fiddlin)
Back hinged section I left totally alone.
Seems to sit horizontal with an even gap along the top of the frame spar.

Spacer:
Actually, anyone wanting to try may look to complete my original idea with the spacer.
Was supposed to slot into the female grove of the bottom half of the airbox, I just could not make it fit, with judicious filing it was barely 0.000005mm from fitting, but could not get it to run correctly the whole way round, hence the draft excluder gasket.
But, upon consideration, I think some softening of the spacer with a hot air gun or similar should make it pliable enough to work into the groove, then you wont need the gasket.
Constant product improvement as they say.....

I think, on aprilia performance, that some one in the last month or so has dynoed the effect of raising the tank - and was surprised to see no gain

BUT

I think that was on an open airbox - so is a different animal entirelt!

may be worth a dig around Keith?