[Gpa113]

Re expanding foam, you have exactly the right idea,  do it one go if you can, so much simpler. 

The expanding foam expands a lot, I mean A LOT    So that wouldn't be a good product to build up a nice flush finish to your track.
I used a roll of 7mm camping foam matt, for sleeping on. Technically 7mm thick XPE foam.
I used paper templates to get the correct sizes/dimensions,  and then just the cut the foam matt to size. Glue in place with a tub of glue designed for scenic flock matt, or whatever works for you....

Very cheap, just buy the cheapest you can find,  unless you actually plan on sleeping on it 
Having done this once, my advice would be cut it slighter larger than perfect,  so that if it shrinks a little during glueing,  or more likely painting,  you will avoid the small narrow stripes in places where it has shrunk, albeit,  very marginally.  I filled these little gaps with silicon, then PVA on top, with flock or scatter applied.  This actually looks really effective,  as it mirrors areas where drivers have cut the corner in pursuit of that all important extra tenth of a second.  Particularly true for a rally stage inspired layout. Happy to send photos if that helps.

Pip