I am planning on building some solar room heaters, probably next year. If you have a south facing wall, they are reasonably simple to build...
here is an example. i plan to mount mine directly to the vertical wall with an intake coming through the wall near the floor and outlet closer to the ceiling. A low speed ceiling fan might help circulate but not 100% necessary.
http://www.mobilehomerepair.com/article17solar.htm
We also have an attic bedroom that is inaccessible via furnace, so i hope to do a fan powered solar heater for these, with a small electric fireplace as a backup for grey days.