Printing an alist of plists with cons format

Have made a two level plist 

(defvar mpl-length
  '(:cm  (:10pt 283.46456 :pt 28.34646 :mm 10 :in 0.39370 :ft 0.03281)
    :in  (:10pt 720 :pt 72 :mm 25.4 :cm 2.54 :ft 0.083333)
    :ft  (:10pt 8640 :pt 864 :mm 304.8 :cm 30.48 :in 12)))

And wanted to convert it to a different form: making the top-level plist
into an alist.

(defun mpl-plist-to-alist (plist)
  "Convert a nested two-level plist (:u1 plist1 :u2 plist2 ...)
to a plist nested in an alist '((u1 . plist1) (u2 . plist2) ...)."

  (let (alist)
    (while plist
      (let ((unit   (pop plist))
            (plist  (pop plist)))
        (push (cons unit plist) alist)))
    (nreverse alist)))


This gives 

((:cm :10pt 283.46456 :pt 28.34646 :mm 10 :in 0.3937 :ft 0.03281) 
 (:in :10pt 720 :pt 72 :mm 25.4 :cm 2.54 :ft 0.083333) 
 (:ft :10pt 8640 :pt 864 :mm 304.8 :cm 30.48 :in 12))


How can I get the new list being printed as

((:cm . (:10pt 283.46456 :pt 28.34646 :mm 10 :in 0.3937 :ft 0.03281))
 (:in . (:10pt 720 :pt 72 :mm 25.4 :cm 2.54 :ft 0.083333))
 (:ft . (:10pt 8640 :pt 864 :mm 304.8 :cm 30.48 :in 12)))