@3RLivestockandFowl
Yes, you absolutely need the back door. The heater won't warm up the chamber interior if there is no door. This may be dangerous as the heater will possibly overheat without the fan circulating air around it and through the chamber.
The back door (airbox) on my unit was wood, probably MDF, sheathed in something like Formica for humidity resistance and aesthetic reasons. I think you can buy MDF with a plastic coating applied for use in shelving. This might be an option.
To be honest, you could probably use unsheathed 1/2" plywood, but it would have to make good contact with the back of the unit to prevent large air leaks.
If there are too many air leaks, your humidity will drop and you will be running the boiler / humidity generator constantly to keep up. Use self-stick weatherstripping on the airbox edge to make a seal with the chassis.
You might want to put a coat of paint on the inside of your new airbox since it will be subject to high humidity. Don't let paint fumes get into your incubation chamber.
One of the videos on my website shows the airbox of my old CP50 pretty well. Note the location of the humidity hose fitting on my airbox. They are standard basement-sump-pump fittings that you can buy at Home Depot. I got my humidity hoses there too.
Here is a link to my video:
Hatchrite Rebuild Video #3 of 3
The airbox is shown at about 4:28 on this particular video. There are other videos you can watch that shows the rebuild process of my Hatchrite and the building/explanation of my humidity generator.
Other stuff: Make sure your new airbox is deep enough to clear the fan and heater by a few inches. There may be witness marks on the back of your unit, showing its original airbox size and the location of the its hinges. That may be useful to your fabricator.
Good luck with your Hatchrite!
