My instinct would be to go for the cedar. Poplar does not fare well around water, rotting very easily. That said, 300C is almost 600 degrees fahrenheit. Just what in a sauna is that hot?

No poplar. No poplar. No.

No.

No.

Over a 100 years ago Poplar wood contained an oil
that prevented water and insect damage.
It made a perfect wood for house siding and fencing.

Poplar wood of today does not contain that oil.
It slowly lost that property over the years. (I forget why)

Now in a wet environment it will rot very quickly.
And unless quarter sawn it will warp and cup very badly.

Pooh Bear