What follows is the method I use to create louvers.
While what is described here may seem a difficult task, it is actually a simple technique requiring only patience and thought, and is well within the capabilities of the average modeler.
As with any new method/technique learning start with an easier type pattern. You should even do a few "practice runs" before cutting away a kit piece.
This method is more suitable for the larger scales (1/25, 1/24th) but has been used in the smaller scales (1/35th and 1/32nd).
Nothing special needed just common modeling tools.
Step 1. Gather together all the reference material (photos/plans etc). Armed with this info its time to do some math: work out the required spacing, louver width and gaps etc.
Step 2. Mark out on some flat sheet the area to be louvered. Normally I will use 0.25mm (.010") as the backing sheet. Allow extra space all around the actual hood panel (10 -15mm (13-19/32")) this is for ease of handling. I use Vernier calipers for the measuring and lightly scribe the lines with a fresh #11 blade tip (a moistened finger tip rubbed across the scribed area will help the lines show up). The reason I use Calipers and scribe the lines is because it is a far more accurate method than working from a pencil line (which could be .5mm (.020") wide) and the spacing may only be that same width.
Image 2 - Mark Out
Step 3. Optional: If you want to hollow out the louvers, it is best to cut slits out of the backing plate as this makes it much easier later on. Just remember to make the slits slightly narrower and shorter than the actual louver.
Image 3 - Slit and Lay
Step 4. Next cut the required number of louvers from some 1/4 round strip, plus some spares (in case of mishaps). For 1/25 and 1/24th scales I use 0.75mm (.030") 1/4 round strip, however you can use any suitable size square or rectangular strip, it just requires a little more work to sand to the required shape to suit the vehicle or scale.
Glue down one louver, starting from the bottom row (or louver closest to the cab), this is to give room for the shaving and sanding. Allow the glue to dry, carefully contour the ends to the required shape. Glue down the next louver and repeat the shaving and sanding process. To make this step goes a little quicker, just alternate between the left and right panels. Continue these steps till all the louvers are in place.
Once they are all glued down and trimmed, give the area a quick blast of primer. This helps to seal them in and also show any defects. Lately I have started to take a photo when I reach this point; when the picture is displayed on the computer screen it really shows any defects from the contouring stage. Try to get the ends as uniform as possible.
Image 4 - Lay and Prime
Step 5. If you intend to hollow each louver then you will need to build a simple jig to hold the work piece. Although not essential, I have found that it makes it easier when handling and reduces the chance of damaging the rather flimsy piece. My jigs are just made from scrap plastic card consisting of a 1.0mm (.040") base plate then some 0.25mm (.010") risers and capped with some 0.75mm (.030") top plates. The base plate has a slot cut into it. For the actual hollowing out use a fresh #11 (or your preferred type) blade and with just the tip, very carefully start hollowing each louver. Do not try and remove large amounts with each cut, just shave off slivers or you will end up tearing the work piece. Work slowly and carefully and don't try and do the whole lot in one session. Normally I'll hollow out 4 or 5 then put it aside for a while then do a few more and so on till they are all done.
Image 5 - Jig
Image 6 - Hollowing
Step 6. Give another blast of primer then check for any defects and clean-up where necessary. Trim off the excess plastic and attach to your model.
Image 7 and 8 - Tidying
Image 9 - Almost Done
I have been using this technique for a number of years now and consider that the time and effort required to be well worth it; as the results are more than pleasing and creating a unique and realistic aspect to a scale model.