For further inspiration about a minimalist racer that is simple and fast to build do a search in the 1/2A forum for "swr" which stands for Sheet Wing Racer. Some of the guys are having a great time with simple to carve sheet wing models.

For a 25 I could see a simple bagged foam or all sheet carved wing model of about 280 to 300 sq inches with an all up weight of around the high 20 to low 30 oz mark being a real scourge around the course. Probably light 3/8 or 1/2 inch balsa with a spruce sub spar to help out the streght issue.