Door Thermometer and gasket, and insulation.

[JRPizza]

That door is going to be pretty heavy with a half inch of steel! I used 16 gauge mild steel for the outer portion of door and 20 gauge for the inner door (.051", .032") and my door weight around 11lb. I'm guessing you will be around 30+ with quarter inch plate.

[danjmath]

I'm finally building my door this weekend, and am wanting to make it cheap as possible after going overbudget on the over(surprise)
I've got some 1/4 sheet steel I plan on using for the inner and outer door plates, and some thinner stuff to use to close it.

I plan on making it 3 1/2 inches thick, (3" between doors + 1/2 inch of steel).

I was going to use leftover loose perlite to fill it, but alas, the outer p-crete layer took ever last crumb I had.

So my questions are thus:

What should I use to insulate the inside, since I have no leftover perlite or ceramic blanket? Is there a noticeable difference in insulation characteristics, or is whatever I can get cheapest work?

What do I need for a gasket? Does self-sticking stuff similar to this:
https://www.amazon.com/dp/B0190YY796?psc=1
work? Or should I get something else?

Finally, I plan on putting a thermometer in the over door, just so I can more easily see the temp the day after. Is there anything special about the forno bravo thermometer, would it work (at least temporarily) to just use an old meat-type thermometer I have with a 4" probe (goes up to 700)?