I have a ash pan on my 5" gauge 0-4-0 Gemma that is closed in the front and sides, but is open at the back with a door hinged so I can open or fully close the rear opening. I have it fully open when lighting and getting up steam but once on the track find I can close it completely as enough air comes in around the edges. The ash pan is made of stainless (about .025 thou thick, with the front silver sol. in) and is about 3/4" deep. It has survived well for the 25 years it's been in position (a little warped now though). About every 30 min. or so when stopped at the station I use the poker to rake out the accumulated ash (the track through the station area is over concrete,so no fires). By doing this I have never had it clog up. The original SS built up grate is still in excellent condition with only a bit of discolouration.
When I was in the Winnipeg club, we had, during the summer 20L pails of water strategically placed around the track as there were a couple of engines there with open bottomed ash pans (and one with no ash pan!) as invariably some one would drop a hot coal and start a grass fire, and someone would have to run across the field, grab the nearest pail and put it out!
John Purdy