If you were determined to build a shelter to protect you from wildfire I would stat with the simple bermed or underground shelters designed against aerial bombing, gas attack and firestorms.

The walls and ceiling are thick enough to stop blast and splinters should go a long way to being insulated and impervious to the forces fire bring. The ventilation systems of most shelters lacked filters because experience in WWI showed simple isolation was sufficient, far more reliable and cost effective than filters when facing an unknown contaminate. The external vents and doors were made to be gas-tight when shut down. The internal volume inside was calculated to be sufficient for the occupants for a few hours. Enough time for any agents to settle or dissipate.

This basic design concept, as shown by the military run shelters in Hamburg and Dresden, was sufficient even as a firestorm raged overhead. Unsealed shelters, typically basements, didn't fare as well. They provided good protection against bomb blast and splinters and fair protection against actual fire, they provided multiple escape routes and generally burned slow enough to allow sufficient time to evacuate, they were vulnerable to carbon monoxide and toxic gas buildup. Thousands of 'sleeping' bodies were recovered.

Further protection could be had by just keeping the shelter away from stands of trees and buildings that might burn and radiate heat, or potentially block the entrance, and limiting the amounts of flammable materials that reside on top of or near your shelter. Short grass or clover shouldn't be a problem as they would burn off quickly.