The bed doesn't really need insulation to get hot fast, but it will prevent the heat from impacting the surroundings too much.
The more important bit is to ground the bed properly. This is an AC heater and may injure or kill you if you are exposed to the voltage. Grounding means that if something shorts out then it will go to ground and blow the fuse/breaker.
One important thing to remember is the aluminum may have a clear coat or some oxidation on it preventing a good ground without some additional measures. Here was my first ground attempt, note it is reading as an open circuit, not good.
I then made 2 changes. One I added a star washer to the grounding connector so it would dig into the aluminium.
I also sanded the spot where I attached it lightly. And voila, a good ground.
For the photo I'm just checking the aluminum piece, but you really must check the edge of the aluminium bed itself. Be sure to press the tip of the probe into the bed(this is why you check the edge) to get through any clear coat or oxidation. I did this and the reading was the same.
Once the ground wire is attached I'll finish insulating the edges.