Hemingway: "to put your hat on even under the canvas at noon"

The person being talked to is in a tent around noontime, which means that it's pretty unlikely he's going to be in the sun (since the sun will be almost directly overhead and blocked by the tent). Acknowledging that he is unlikely to be in the sun, the speaker is asking him despite that shouldn't he put on a hat?

Most of the sentence is pretty straightforward. "Hadn't you ought to" roughly equates to "Shouldn't you" and "under the canvas" means "in a tent". We know it's a tent and not something else made of canvas because the Oxford English Dictionary has the following definition:

under canvas: in a tent or tents.

(This isn't an expression I'm familiar with, despite being a native speaker, so I think it must be an older expression that's fallen out of fashion.)

In addition to this, from context earlier in the story, we are told they are in tents:

It was now lunch time and they were all sitting under the double green fly of the dining tent pretending that nothing had happened.

With a single exception, the sentence has its straightforward meaning.

"Hadn't you ought to" means "shouldn't you", and it's a pretty common vernacular variation.

"Under the canvas" might refer to a tent, but it might also refer to a canvas shade. While "canvas" can refer to the kind of cloth, it can refer to items made from that cloth. While "under canvas" mean "in a tent or tents", the expression doesn't normally include the definite article.

Specifically what the question is referring to is that under very bright sunlight, such as in Africa at midday, canvas does not provide complete protection from the sun and one would be well advised to have additional protection from the sun. The observation about Wilson having a red face would back this up, although it may carry other implications too.

(The subtext in which Mrs Wilson changes the subject from the important to the banal is more literary than linguistic and not really to be covered here.)