Insulating the top will certainly help, but you'll need to insulate the bed too if you really want it to be warmer. I've insulated my Westy behind all of the panels and it makes a heck of a difference in road noise but only a mild difference in interior temperature- that's subjective as I never measured it. With all of the glass (even with curtains) and the fiberglass top, it bleeds heat off really fast. A Mr. Buddy heater warms it nicely, but it looses that heat quickly after you turn it off.
The only way I've been able to make a real difference is to put reflectix over the windows and under the bed and drape a wool blanket over and in front of the bed to reduce cabin space and provide some insulation.
In my old Tacoma, during the winter I had an old wool blanket on top of the carpeted platform and I suspended another inside sort of like mosquito netting- think of it as a wool tent inside the bed. The fiberglass cut the wind, but the wool actually insulated some. The best I've found for this purpose, heck the best wool blanket deal I've ever found period, is the surplus Italian wool blankets. I'm not sure what they're selling for now, but they were around $10 for a really heavy 100% wool blanket that is about 64"x80". They have some sort of moth repellent on them but if you hang them outside for a couple of sunny days it goes away. I usually wash mine with Woolite then hang them outside. After they dry I tumble them in the dryer for a bit to soften them up.
05 Tacoma TRD, SR5, V6, etc
1978 VW Westfalia
2007 Suzuki DRZ400s
Who is the happier man, he who has braved the storm of life and lived or he who has stayed securely on shore and merely existed?
Hunter S Thompson