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The premise of the build revolved around a 6" section of PVC and an air filter from a F450/550.
Parts required:
1x F450 air filter
1x 8" section of 6" PVC
2x 6"x4" Fernco rubber couplings
2x 3" Fernco rubber couplings
1x 4"x3" PVC reduction bushing
2x 3" PVC 90° bends
1x 3" PVC 45° bend
1x 12" section of 3" PVC
1x 24" section of flexible 3" intake hose
1x 3"x2½" intake coupling
1x Black door trim
1x Precleaner for the intake head. I used a Donaldson TopSpin that I had acquired long ago.
The flange on the filter seats perfectly on the 6" PVC...
I got a little carried away and forgot to take pics of the next several steps, but here's what happened:
The PVC bends I used were male on one end and female on the other (street bends). For plumbing code and whatever else reasons, the ends are designed to accept 2" of male tube into the female tube. Wanting to keep the size as small and compact as possible, I shaved 1" off of each end of the pvc fittings...1" of fused seal will be ample for the needs of an intake. I sanded the flared end of the reduction bushing to make it smooth and then cut the reduction bushing in half to yield 2 pieces. I also shaved 1½" off of the 4" end of the large Fernco couplings. I cut the 6" piece down to size, inserted the air filter into the pipe and capped it with the Fernco couplings (you might wish to mark it with an arrow indicating air flow as not to forget). When done, the pieces looked something like this:
Coming out of the air canister, I used the two 90° bends. I inserted the male end of the 3" bends into the reduction bushing and glued it up. From there, the reduction bushing fits in the 4" end of the Fernco couplings perfectly and the band clamps ensure an airtight fit.
I attached the 3" flex hose to the intake using the coupling and gauged where to route it. Coming out of the engine compartment, I went through the firewall and into the passenger area behind the dash. For both of the holes, I used a 4⅛ hole saw. Run a strip of the door trim around the hole to avoid any sharp edges which could cut through the 3" Fernco coupling and then install the coupling. This finishes the work under the hood.
Now put one of the bends into the air can and tighten the band clamp. Put a short section of the 3" pvc (4" or so) into the female end of the elbow. For the next step, it is much easier of you remove the passenger seat and glove compartment.
Contort yourself like a pretzel so you can now gain access up underneath the dash and insert the air can into the 3" coupling coming through the firewall. Do not tighten this band clamp yet as you will need to be able to move it around to get the correct fitment.
Put the other 90° bend into the opposite end of the air can and then put the 45° bend into the 90° (Do not glue these yet). With the bend loose and not glued, you can twist them until the result looks something like this:
Now you know about where to make the hole in your body cowl. Use as much time as required before deciding where to make the hole...you get one shot at this.
Clean the area where you want to make the hole (it is not necessary to clean any other area of your Jeep!).
Mask the area w/ tape and take the plunge...
Use another piece of the door trim to line this opening also
Insert the remaining 3" coupling into the opening and set the height of where you want it to be...
measure, cut and fit the remaining 3" pvc between the rubber coupling and the 45° bend. Once you're satisfied w/ the angles and fitment, mark the bends, remove and glue them up.
If you want to paint it, now is the time. I used a leftover can of bedliner I had in the shed...
Putting it together. Assemble the air can using the filter, 6" pvc and Fernco couplings. Remember which way you installed the filter so you get the flow direction right...
Now install the other pieces...
and then fit it all together in the Jeep...
Install the prefilter to the topside of the cowl coupling and secure...
If you didn't think to check the hood clearance before cutting the hole, you can do that now also...
re-install the passenger seat and glove compartment...
and you're done. At this point, I fired the engine to hear it breathe. I couldn't detect any leaks and the engine seemed to be getting plenty of air. For added piece of mind, I removed the prefilter and covered the intake opening with my hand. I was able to shut the engine down, satisfying my questions as to how sealed it actually was...and it's sealed!!
Haven't put it in a real world testing situation yet and I'm really not chomping at the bit to do so; but, if that situation were to unexpectedly present itself...I'm pretty confident that it would pass.
-dave
Give me a call when you're ready to give it a"real world" test. 
