Couple questions, how old is your house approx., what type of heat do you have, what are the current window covering in that room, and what is the flooring?

I do not like space heaters. If this is not a budget project, I first would look into adding another supply run to this room. Second would be a small strip baseboard heat with understanding it can be expensive to operate.

IMO

With only a 5-6 degree difference, you may be able to either balance your heating system better and/or add insulated curtains and avoid the extra heating bills. If you have forced air, you may be able to adjust your dampers to allow a little less heat to the room with the thermostat and/or in the same supply run as the cold room, and keep the cold room wide open to try to get them all better balanced. Thermal insulated curtains can do wonders for a cold room. Also, there is everyone's favorite...the plastic shrink film you can apply on the inside of the window (or install a real internal "storm-type" window that is removable for the warmer months and doesn't require applying double-stick take all around the window).

I bought a very cheap wall mounted electric fire place on amazon for the kid cave in our unfinished basement. It mounts into a tv like mount, plugs in and heats well to set temp. A little cheesy looking but we love it on movie nights. It is the only heat source for the basement and it works sufficiently.

a wall mounted mini split heat pump is a greater upfront cost but would save money long term on energy costs.

Vornado makes nice space heaters. Fan-forced, the heating element is concealed inside.

Finally got around to thinking about this project again.

The house is from the late 50's but the room was added as part of an early 70's addition. It's over a crawlspace, the original home has a full basement. The window is builder's grade and it's covered with some kind of roller shade. Gas, forced hot air heat.

• I did play around with closing vents, helped just a bit.
• I did purchase a cheap space heater. On the low setting (~1,000 watts) it keeps the room plenty warm but it's fan is very annoying.
• The floor of the room and adjacent kitchen is always cold. I've thought about insulating it... Not quite sure how to approach that.
• As I said originally, this room was somewhat unused and once we get back to normal will probably not be used a lot again.

I'm back to a simple electric baseboard heater (and maybe some insulation).

Insulating a floor over a crawl space can be done but I don't recommend it. The easiest way is fiberglass batts, but mice love to nest in it. 2 inches of rigid foam under the subfloor isn't bad but can be a nightmare to install depending on electrical and plumbing. It's better to add insulation to the crawlspace walls and rim joist. How much space do you have to work in?

One supply to room, what about a return for the room?...how practical would it be to add another supply?...

When adding heat, you need to balance the capitol cost to install the heater vs. the operating cost. A straight resistance heater is the cheapest to install but the most expensive to operate. Since you don't plan to need this supplemental heat forever, perhaps an electric space heater might be the best. I happen to like those oil-filled heaters that look like radiators - no fans. And the surface temperature never gets dangerously hot.