Depending on the price of pre-fab ones, I'd be tempted to make one.
Not just from being cheap (which I am...) but I suspect the 30A male plug isn't going to last long, for the reasons others mentioned above. I'd rather replace an end, than scrap an entire dogbone.
When plugging in at a campground, I try to avoid plugging my shore tie (30A, molded male plug, coach end permanently connected) into the 30A female campground receptacle. If I can, I use a 50 to 30 dogbone that I've replaced the female with a box that has a pedestal-style receptacle in it. The molded female 30's are JUNK. Bad enough that the pedestal and the shore tie male get trashed, but the molded females are the WORST. Their contacts go bad easily then the male hot blade overheats and starts its toll on that plug.