Don't mean to be rude.. but.. if Fred Flintstone had a sup board that would be it.. I know someone who once made their first board.. He called that board Mr Lumpy.. Your board reminds me of that board..

You might find that the paddling straight thing is often more a technique thing rather than a fin thing.
Try standing slightly more forward so the tail is just starting to lift from the surface of the water.. Try taking shorter paddle strokes and don't bring the paddle much past your feet.. and try putting more weight on the same rail side that you are paddling on.
If this fails you need to remove your fin and put in normal fin box so you can try different fins till you find one that works.. Adding side fins might help.
Just checking.. It's not April the 1st is it?..

.. Do you have a pic of your paddle? .. I can just imagine what it's like..

DJ