Shorting SGOV over box spread

I have a minor margin balance at 11% interest, and was looking for alternatives to save a small amount of cash on fees. I noticed that shorting stocks increases my margin debit balance, which I thought was strange because I somehow thought short stock also reduced your buying power. Thinking shorting SGOV and just paying out the 4.5% dividend then covering when I no longer have a margin balance might work for this use case, over a box spread. Obviously things that are too good to be true often are and there is no free lunch, so was looking to poke holes in this theory.