i honestly dont like computer cases for the enclosure. but it is all personal preference.
you want something that will have a very sturdy place for the lasers dichros and scanners to mount to.
otherwise you will have to deal with things getting out of alingment and you constantly having to adjust the dichros and mirror mounts.
i have seen some people use horizontal pc cases very successfully but they also used a thick aluminum plate as the base plate.
the scanpro 20 amps are VERY small. heres a picture of my micro 500mw green build.
the scanner amps are in the top left hand of the enclosure and the size of the entire enclosure is 10"x6"x3.5"
here is another picture of the scanners sideways
as you can see they probably go half way across when mounted side by side so thats about 3 inches but something that isnt shown in those pictures is the way i mounted them, scanner amps are prone to getting very warm-hot and you want them as cool as possible, i have mine mounted to a nice thick piece of aluminum to help them cool.
you will want to do the same, but most likely you will have a thick baseplate you can mount the the amps to so that isnt a problem most people have.