befuddled user seeks help with defmacro

[Eric Moore <moore@chem.cmu.edu>]

Ok, I was working with guile, and wanted to make an easier means for
non-lisp saavy users to create and set some smobs I'd created.  I hit
on a solution that looks more or less like (rectangles have been
substituted as they're the canonical example ;):

(defmacro define-rectangle (rect-name . body)
  `(let ((rect (make-rect)))
     (define ,rect-name rect)
     (let ((height: (lambda (str) (set-height! rect str)))
           (width: (lambda (str) (set-width! rect str))))
       ,@body
       )))

with the intent being that something like:

(define-rectangle fred
  (height: 10)
  (width: 12))

would create a top-level binding of 'fred' to a 10x12 rectangle.

Unfortunately, it doesn't work.  I tried replacing the define with

     (define (quote ,rect-name) rect)

and

     (define ,(quote rect-name) rect)

still no luck.  However:

(defmacro define-string (string-name)
   (define ,string-name "foo"))

does.

I've read all the docs on quasiquote and defmacro I can find, and
examined the source to defmacro, and came away unenlightened :)

so I have a few questions:
a) is there a better way to do this?
b) why does it work this way? 
c) how could I make it work? (even if the answer to 'a' is yes, I'd
   like to understand defmacro better ;)

Thanks in Advance for your help!
        -Eric