This prop rod kit from Big Al's Carponents allows the trunk to automatically lock in the open position.  It features a manual close device.  The prop rod is machined aluminum 1/2" tube and brackets, with 5/16" stainless rod, latch device and hardware.  It is recommended for use on 37-38 or earlier cars due to the weight of the trunks.  It is not recommended for a hood prop due to the weight of the hoods.  Closed tube length is 13", while open tube has two positions that must be set upon installation.  Middle open position is 19-1/2" and maximum open position is 21-1/2".