Hmm most of these suggestions aren't muff clones. The ones that come to mind right away are the LBM which is known to work well with bass, the Pickle Pie 'B' by Wren and Cuff, Skreddy Mayo (good luck finding one though), GGG or BYOC muff. You could always mod your own for bass too...hope this helps.
Are you decent with a soldering iron? I built the GGG muff which sounds great on bass. They have two case options for it. I opted for the larger one which isn't that large at all compared to the original BMP. It's about the same size as the Little Big Muff.