Although it may not be morally ok, one could just buy a laptop for a conference etc, and then return it after a week if the store has a return policy. I've seen stores take back laptops no questions asked for up to 60 days.

If you are in a pinch.

If not you can just sell it (there or at home) and take the loss as a rental fee.

Why wouldn't a rented laptop be trustworthy?

Because it's someone else's hardware, someone else's software, and someone else's firmware.

To be pedantic, unless you flashed coreboot it's not your own firmware and if you aren't using an open hardware laptop then it's not really your hardware either.

But you can have a bit more trust in the supply chain to get to you than that it hasn't been tampered with afterwards.