Possibilities...

Lower wattage lightbulb, greater distance from bulb to paper, and graded photopapers (but may not be available in postcard papers) -- as previously suggested.
Neutral density filter -- one or two layers of smoky glass from glass shop...or even stretched-out pantyhose.

Your 30 sec development time in Dektol is very short...try about 90 seconds to allow fuller development and even development (plus the longer times will make it easier to be consistant)...but the print you show does have a wonderful feel to it. A greater amount of development will probably cool the print down some, also. One can dilute the Dektol to get similar results with longer development times.

Vaughn