How do I say 'people insist on' in the passive voice?

If I have 'read guides where people insist that...' then how do I use that in the passive voice?

'In guides I have read, it is insisted that...'? 'In guides I have read, it is insisted upon that..'?

Or is it something different?

Here X is the thing insisted. For example, X could be "visitors to Italy must try the delicious local pizzas".

You could say, "There has been insistence that X". Or "That X has been insisted".

And your structure also works, "... it has been insisted that X".

You can prefix or postfix any of my examples with "In guides that I have read", "In some guides", and so on. For example:

In guides that I have read, it has been insisted that X.
It has been insisted that X in guides that I have read.
In some guides, there has been insistence that X.
It is insisted, in some guides, that X.

These are all acceptable, though some are awkward.

If you really wanted to, you could say it is insisted that and you would have the support of at least one citation from the OED (Oxford English Dictionary). It is insisted upon that, however, has no such support. In any case, both sound very formal and are unlikely to be used in contemporary English.

It depends on the exact sentence, specifically on whether the construction is "insisted that" or "insisted on"; your question mentions both.

Here are two examples:

I have read cookbooks where people insist that you sift your flour twice.
I have read cookbooks in which it is insisted that flour be sifted twice.

and

I have been to countries where people insist on eating dinner at three in the afternoon.
I have been to countries where eating dinner at three in the afternoon is insisted upon.