How to cut a countertop to fit exactly within a closet?
Yes, you definately need to make a two-part template first. One half of the template will have the odd-shaped portion, and the other half can overlay it. Use tape or hot glue to secure them together.
Fasten the template to the countertop. Use a jigsaw to rough-cut the countertop to the template, leaving about 1/8"-1/4" extra material. If you have a router or a friend with a router, you can use a bearing pattern bit to flush trim the remaining 1/8". The router bit's bearing rides the edge of the template and cuts the countertop to exactly match the template.