Full length top rail similar SPR (PRI, ARMS?) one?

[BCP]

I have an 18" rifle build using a midwest free float tube and would like a full length top rail system like the SPR rifles have using the PRI tube and PRI? rail. Is there a rail like that out there that can mount to any handguard provided that handguard has a picatinny rail on top? Or should I just sell the midwest handguards and spend the damn near 500$ f***ing dollars for the PRI float tube and PRI rail?

edit:

This is the handguard I have in rifle length:

http://www.midwayusa.com/product/2344114...able-rail-ar-15

and I want a top rail for this sort of look:

http://www.hunt101.com/data/500/IMG_8477_1.jpg