The Slant-Fin Multipak 90 and R series flat-top baseboard covers can take commercial fin-tube that will exceed the output from an 18-foot space. The 90-14 cover with the C340 element will give the required output. The cover is 14″ high with a sloped top, so it might be less attractive than standard baseboard small size but it will do the job. The 95-10 with the C340 element is ten inches high with a flat top that might be a choice. You require about 1100 btu per linear foot at 180F, so this is a large-capacity requirement. Other manufacturers make commercial baseboard as well.