How can I add jars from more than one unmanaged directory in an SBT .scala project configuration

I'm trying to get SBT to build a project that could have more than one unmanaged directory. If I had a single directory, I could easily do it like this:

unmanagedBase := file( "custom-libs" ).getAbsoluteFile

But since I have two directories with unmanaged jars, I need to be able to add them all. I have found some information in here, but still doesn't seem useful for my full .scala file build.

I have created a simple project that shows the issue in here. And below is my Build.scala file.

UPDATE

I've got some help form the sbt-users list and have been able to define the unmanaged-jars correctly, but the code still doesn't compile (but sbt show unmanaged-jars shows the files correctly).

import sbt._
import com.github.siasia._
import PluginKeys._
import Keys._

object Build extends sbt.Build {

  import Dependencies._

  val unmanagedListing = unmanagedJars :=  {
    Dependencies.listUnmanaged( file(".").getAbsoluteFile )
  }

  lazy val myProject = Project("spray-template", file("."))
    .settings(WebPlugin.webSettings: _*)
    .settings(port in config("container")  := 8080)
    .settings(
      organization  := "com.example",
      version       := "0.9.0-RC1",
      scalaVersion  := "2.9.1",
      scalacOptions := Seq("-deprecation", "-encoding", "utf8"),
      resolvers     ++= Dependencies.resolutionRepos,
      libraryDependencies ++= Seq(
        Compile.akkaActor,
        Compile.sprayServer,
        Test.specs2,
        Container.jettyWebApp,
        Container.akkaSlf4j,
        Container.slf4j,
        Container.logback
      ),
      unmanagedListing
    )

}

object Dependencies {
  val resolutionRepos = Seq(
    ScalaToolsSnapshots,
    "Typesafe repo" at "http://repo.typesafe.com/typesafe/releases/",
    "spray repo" at "http://repo.spray.cc/"
  )

  def listUnmanaged( base : RichFile ) : Keys.Classpath = {
    val baseDirectories = (base / "custom-libs") +++ ( base / "custom-libs2" )
    (baseDirectories ** "*.jar").classpath
  }

  object V {
    val akka    = "1.3"
    val spray   = "0.9.0-RC1"
    val specs2  = "1.7.1"
    val jetty   = "8.1.0.v20120127"
    val slf4j   = "1.6.4"
    val logback = "1.0.0"
  }

  object Compile {
    val akkaActor   = "se.scalablesolutions.akka" %  "akka-actor"      % V.akka    % "compile"
    val sprayServer = "cc.spray"                  %  "spray-server"    % V.spray   % "compile"
  }

  object Test {
    val specs2      = "org.specs2"                %% "specs2"          % V.specs2  % "test"
  }

  object Container {
    val jettyWebApp = "org.eclipse.jetty"         %  "jetty-webapp"    % V.jetty   % "container"
    val akkaSlf4j   = "se.scalablesolutions.akka" %  "akka-slf4j"      % V.akka
    val slf4j       = "org.slf4j"                 %  "slf4j-api"       % V.slf4j
    val logback     = "ch.qos.logback"            %  "logback-classic" % V.logback
  }
}

As answered by Eugene Vigdorchik, what made it work as the following code:

import sbt._
import com.github.siasia._
import PluginKeys._
import Keys._

object Build extends sbt.Build {

  import Dependencies._

  var unmanagedListing = unmanagedJars in Compile :=  {
    Dependencies.listUnmanaged( file(".").getAbsoluteFile )
  }

  lazy val myProject = Project("spray-template", file("."))
    .settings(WebPlugin.webSettings: _*)
    .settings(port in config("container")  := 8080)
    .settings(
      organization  := "com.example",
      version       := "0.9.0-RC1",
      scalaVersion  := "2.9.1",
      scalacOptions := Seq("-deprecation", "-encoding", "utf8"),
      resolvers     ++= Dependencies.resolutionRepos,
      libraryDependencies ++= Seq(
        C.akkaActor,
        C.sprayServer,
        Test.specs2,
        Container.jettyWebApp,
        Container.akkaSlf4j,
        Container.slf4j,
        Container.logback
      ),
      unmanagedListing
    )

}

object Dependencies {
  val resolutionRepos = Seq(
    ScalaToolsSnapshots,
    "Typesafe repo" at "http://repo.typesafe.com/typesafe/releases/",
    "spray repo" at "http://repo.spray.cc/"
  )

  def listUnmanaged( base : RichFile ) : Keys.Classpath = {
    val baseDirectories = (base / "custom-libs") +++ ( base / "custom-libs2" )
    (baseDirectories ** "*.jar").classpath
  }

  object V {
    val akka    = "1.3"
    val spray   = "0.9.0-RC1"
    val specs2  = "1.7.1"
    val jetty   = "8.1.0.v20120127"
    val slf4j   = "1.6.4"
    val logback = "1.0.0"
  }

  object C {
    val akkaActor   = "se.scalablesolutions.akka" %  "akka-actor"      % V.akka    % "compile"
    val sprayServer = "cc.spray"                  %  "spray-server"    % V.spray   % "compile"
  }

  object Test {
    val specs2      = "org.specs2"                %% "specs2"          % V.specs2  % "test"
  }

  object Container {
    val jettyWebApp = "org.eclipse.jetty"         %  "jetty-webapp"    % V.jetty   % "container"
    val akkaSlf4j   = "se.scalablesolutions.akka" %  "akka-slf4j"      % V.akka
    val slf4j       = "org.slf4j"                 %  "slf4j-api"       % V.slf4j
    val logback     = "ch.qos.logback"            %  "logback-classic" % V.logback
  }
}

Source repo with the full example available on Github.