I recently replaced my radiator and hoses and , as many have found, could not find a part replacement
Here is what I used. GE Silicone II caulk, copper pipe insulation foam for both sides of the radiator, Frost King 3/4 x 7/16 rubber foam, glued together for the upper and lower foam replacements. I used Silicone caulk to bond them together not at their glue side. I used a slight coat of 303 3M Plastic Protectant (which I had anyway) on the foam sticky side to prevent it from prematurely sticking to the car when I was positioning the radiator during insulation. For the Frost king foam, if glued together they approximate the old foam for upper and lower lower dimensions. The Silicone is supposedly heat resistant to about 400 degrees F. Tape was used along all fins to prevent Silicone from getting on any fins during the gluing stage.
I don't know if the foam will last as long as the OEM foam, but, that remains to be seen. Everything seemed to fill the upper , side and lower gaps fairly well.

 

Thanks for posting these great tips on your foam replacement...I appreciate your ingenuity and gumption! I never thought of using pipe insulation in this manner, but I have some left-over from a plumbing job. I'm glad I didn't throw it out!

 

Follow up to my foam insulation on both sides of the radiator. Today, I checked the foam and the pipe insulation quarter rounds that I cut and "glued" to the left side of radiator ( when standing in front of the vehicle) . The GE Silicone II clear caulk did not adhere as well as I hoped. there is a pressure fit along the side of the radiator, but the top 3 to 6 inches of the foam became unglued, but only on the LEFT side of the radiator. The other foams top and right side looked OK. I don't know if the foam did not adhere perfectly due to the heat of the radiator or the air current blowing thru, or I accidentally dislodged it during radiator installation, but if I were to do this project again, I would have used the product below which is meant to withstand -65F to 500F temperatures..... I may go ahead and get some of this stuff and see if I can squeeze some of this in between the existing foam and call it a day. So far only the top 6 inches or so look like its not quite adhering well. I did use 90% isopropyl on the radiator prior to putting on the original silicone....... Good luck guys

PERMATEX® ULTRA BLUE RTV 3.5OZ gasket maker item # 81724

 

My guess is that your original 400 degree adhesive didn't hold for reasons other than heat. Your radiator will never get this hot. Adhesives are a real science. An expert could tell you what type of adhesive to use for this particular application, but I couldn't. At least we know now what doesn't work! Thanks a million for your followup.

 

How necessary is this foam though? I saw some degradation on my 2002 LS430's radiator foam when I was cleaning the fins but I figured as long as there is air passing through it and your radiator fans come on with the AC it’s not really needed. It’s just sealing the front surface of your radiator making a high pressure zone for the air to be forced through. I’m not sure that having the foam there really does much.

 

If the foam surround had no purpose, the auto maker would have not spent the money to add it. Add it per spec.

 

The foam forces the air to be sucked through the front of the radiator and not from the sides of the fan. If you are sitting in traffic in Arizona in the summer, every CFM through the radiator counts. Many have gone without the foam and have noticed no difference in their environment.....

 

You said it yourself...the foam directs the air through the radiator. This is opposed to letting the air flow around the radiator.