I have been trying to come up with basically exactly the same thing for myself to cut eps block to put on my cnc to cut my surfboard however I need to be able to cut down 16' x 4' x 2' blocks.

I have been thinking about it for a year or so trying to figure a way to get some cheap linear motion. I was wondering about just using a wheel that runs on the ali extrusion but then thought maybe that would not be enough. then there are some of the methods available on cnc router parts but in the end I always come back to just trying to find some cheap low quality square rails as they will probably be better than all the above and a lot easier to set up.

my plan for the drive was a belt, it should be more than accurate enough

looks like your going for just two axis rather than 4? I would love 4 axis but think I will probably go 2 for simplicity.