Gap needed?

I've been searching for an hour now and can't find what I'm looking for. Maybe I'm just challenged when it comes to searching.

Anyway, I have a simple question...

I plan on using a tub for my BP. My flexwatt and t-stat are on their way. Can the tub sit directly on the flexwatt or do I need to leave a gap so the flexwatt can "breath"?

No gap needed. It will work better if there isn't a gap.

With most of the tubs, the bottoms aren't flat (they have feet or something); so there will be some ventilation anyways. But yeah, you don't need any really.

No gap needed...Just use Aluminum tape to hold down the Flexwatt so the constant rubbing by the tub being moved over it (when you open / close it) won't rub on the Flex directly.

What kind of flexwatt are using? I only ask because sandwiching 3" 10 watt/foot flexwatt is very dangerous, and can cause a fire. I would not use the 3" 10 watt/foot flexwatt without giving the heat tape some breathing room.

I ordered one foot of 11" and 4". I will use the 4" on a smaller tub and the 11" on a bigger one.