Small holes in fabric can be repaired by simply dabbing or spraying on black paint, with this likely less visible or otherwise better than applying tape. From there, you just need a perhaps 8-12-inch wide piece of fabric to cover the light-leaking zipper; and if a better/tighter seal over the zipper is desired glue on some patches of velcro to keep it in place.
If you have problems keeping it warm (with winter coming), hanging blankets or equivalent over the top and 4 sides can be good insulation, besides blocking light.