Best way to heat a small room?

[TimeWasteing]

I've got a small box 4x4x6 made of 1/2inch insulation board during the day with the lights on (led's and some cfl's) the heats not to bad (68-70) but at might it drops too 55-58.


*The small potted plants seem to be doing fine but the 2x2.5gal DWC tomatoes aren't doing to good.

Any idea's on how to heat it?

I'm thinking of getting this heat mat Amazon.com: Milliard Durable Waterproof Seedling Heat Pad 21"x8": Patio, Lawn & Garden but wana see what others think before I order.

Smaller: version Amazon.com: Milliard Durable Waterproof Seedling Heat Pad 24"x5": Patio, Lawn & Garden
Larger version: Amazon.com: Milliard Durable Waterproof Seedling Heat Pad 20"x20": Patio, Lawn & Garden

I've added a 60w incandescent light blub for added heat during the day but at light it doesn't help.