Masonite or hardboard is a high-density board without a resin. It is listed at around 200 to 275 °C for its autoignition temperature. Just for comparison, let's look at similar products.
Medium-density fiberboard (MDF) is similar to hardboard but bonded with a resin, offering a smoother surface and is listed at 200 to 275 °C, so pretty close.
The heavier High-density fiberboard variant is listed with about the same temperatures of about 200 to 275 °C.
Oriented Strand Board (OSB) is listed at 200 to 260 °C.
All the materials have very similar autoignition temperatures, so none of them particularly shines over the other. Using a proper fire safety procedure is highly encouraged, even though the ambient temperature in the enclosure should not reach even close to the 200 °C unless the printer has a freak accident and catches fire itself.
- A smoke detector is a must.
- Some kind of cooling method that keeps the inside at a temperature down is highly encouraged. A simple temperature activated fan might help in this.
- Lining the inside of the box with a thicker aluminium or copper tape can help to even out the temperature over the surfaces faster, preventing the formation of hotspots that might otherwise reach dangerous temperatures. Copper tapes are more expensive than Aluminium tapes but have a better heat transfer coefficient.1
- Including an automatic fire suppression system could be an option.
1 - The RS Catalogue was just chosen for ease of navigation. McMaster Carr does list special heat shielding tape in both thin and aluminium as well as copper and aluminium foil tapes. I am not affiliated to either.