The five CSIA sweeps I know would make an issue with it - these are not guys I'd want to debate with. Also consider, when fireplaces are used for many hours at a time, like during extended power outages, heat from the fireplace can radiate out quite far from the firebox. The most lenient acceptance I have heard is that the wood form can stay in if the ash pit damper is permanently sealed closed and the form is no closer than 8 inches to the floor of the firebox - if closer than 8 inches and then the form should be removed - which is doable but expensive.