Since this is a question I get PM’d about frequently at Club Lexus… I thought I would make a thread with pictures showing how I have modified my airbox and created more air flow and shielded the under-hood heat.
Note: I must give credit for my inspiration to Peter Scott and his excellent page http://planetsoarer.com/ his testing and information made me take the steps I have…
Now, as far as my heat shield… I am STILL using the corrugated cardboard heat shielding. I admit this does NOT look sexy but it WORKS GREAT. At some point I will get around to making the shielding from something more permanent but the important part is that it keeps the hot air away from the intake.
So, I will post each picture along with descriptions. Here goes:
Here is a pic of the stock airbox lid. This is the same part used on 1995 to 2000 Lexus LS400/Toyota Celsior.
Here are a couple of views of the airbox cover after cutting away the front area (part facing the front of the car). Notice I was careful to keep both the front mounting bolt bracket and the snorkel inlet area.
The mounting bracket allows the airbox cover to be securely re-installed (note the bracket is damaged as I sourced it from a wrecked car and it was damaged, it still accepts the grommet and bolt though). The snorkel inlet area provides support for the factory snorkel that helps create part of the cold air box sealing and continues to provide cold air to the air filter area.
Here is a picture of the modified air box cover installed.
Here is another angle of the installed cover. Notice the “pretty” cardboard heat shielding around the snorkel inlet 
Here is the completed install with the cold air snorkel installed completing the cold air box area in front of the filter. You can see some of the high tech cardboard also
Here is the seal around the headlight I removed to allow as much cold air as possible to the front of the modified air box. It averages over an inch all the way around and the area now flowing to the filter is very large and fed directly from cold air entering the grill. Remember this is for 1998 and newer LS400s but the 1995-1997 should be very similar. The 1990-1994 cars have a plastic cover over the headlight plug that can be removed and a plastic cover in the fender that can also be removed.
Here is an earlier LS400 (1990-1994) showing the headlight plug area and inner fender hole after plastic covers are removed.
Here is a view showing the front of the filter on my 1999 LS400 (same for 1998-2000) as seen from under the grill area. this shows how nicely the cold air feeds to the filter itself. I have driven in very heavy rain and the water never gets to the filter. The angle the air travels separates the water and nothing but cold air gets to the filter…. LOTS of it!
NOW for the glamorous part here is the VERY cool heat shielding
The important thing to notice in the following pictures is how the shielding fits snugly around the air box and prevents hot air from getting to the filter. In this picture there is 2 pieces visable. To the left is thick cardboard like found on the back of a legal pad. This is basically a flat piece wedged between the ABS distribution block and the air box area. It snugs up to the radiator side and creates a block to heat from the radiator area.
Here is a view with the air box front removed. You are looking at a piece of corrugated cardboard that I cut to fit around the MAF. With the back of the box installed as shown the cardboard is wedged between the box, the metal under the box and is folded around the MAF. To the right this piece of cardboard meets with the other by the ABS distribution block. It seals against the bottom of the factory cold air snorkel and forms a fairly good seal when the snorkel is installed.
Here, with the snorkel installed, you can see the seal made between the cardboard and the snorkel. Also notice how it fits around the MAF. This for the most part is just a single piece of cardboard cut to fit.