Not sure if it is correct for your cars, but Westerfield makes a Bettendorf fish belly single I beam underframe for Milwaukee cars, kit 12053.  If it is correct perhaps a call to Andrew Dahm at Westerfield Might yield a couple of underframes.  They are for 43’ boxcars which may be too long for the reefers.  I simply copied it in styrene for a second model.  Picture of the underframe without the details.