I like the sound of this "Polyurethane and Polyisocyanurate foam". Working with sheets of insulation and cutting it to fit just seems much easier then guessing about the expansion rate of a foam, which by the way, I too have a lot of experiance working with. It's great for windows and around pipes during construction or at least prior to finish work, but very difficult to clean up off of nice surfaces such as paint or carpet. And it definately isn't coming out of clothes.

Line the bottom of the machine's interior with Por-15 or simular product prior to inserting the cabinet and that should handle the condensation issue. 

First, you will have to measure the depth of area you have from the back of the inner tub to the back of the cabinet (2 1/2"?). You dont have to bend foam board. Make a paper template of the inside of the back of the machine from top to bottom and side to side. Lay this on the foam and cut it out. Some machines it might fit in whole and some you might have to cut it into two pieces to fit it in.
This works best when the machine is on its back though. You might have to carve some little areas out on the foam for little things that sometimes protrude from the back of the inner tub.
You now have the entire back filled with solid foam.

Now what you are doing is building up the bottom, sides and top to form a space to just drop the tub into. Try to get the foam board to fill as close as possible the shape of the inner tub, so that when it fits in, you can hear the foam squeak.

I will do a drawing in the next few days and layout how an 81 would be fitted. I think you will be surprised how easy this is, and custom fit to the shape.

It wouldn't work that well on slider, the evaporative tubing that's attached to the inside on the liner will leave a 1/2" gap all around the sides & bottom.  although it has worrked nice on some chest coolers I've done ( for the bottom to replace the original hemp-board insulation)