The phenomenon of thermal fluctuations of biopolymers has been of active interest for some time with a view toward understanding the effect of filament confinement, migration, and bonding. In this study, we focus our attention on planar fluctuations of a single filament between parallel confining surfaces. Filament slopes, with respect to the centerline of the channel, commonly exceed 0.1 in magnitude and therefore fall outside the range of small amplitude fluctuations. Consequently, large amplitudes are anticipated from the outset. Determination of the partition function leads to the quantitative dependence of free energy and other thermodynamic parameters on the degree of confinement.