Chet,
A possibility: Get a seedling heat mat. They draw 17 watts (120 v) and heat up to about 80 deg.
Put insulated material under it, some cozy material or foot pockets on top of it, and put it where you spend the most time sitting. It won't get too hot or drain your battery quickly.
Another possibility: Make a small hood for the plat cat to draw off the convective heat from above the frame and screen. A small 12 volt fan and some bilge pump (or vacuum) hose will put this warm air exactly where you want it.
Good Luck,
