just Google Styrofoam incubator and you will find many DIY sites explaining how to set one up. To be honest, by the time you buy all the things you need, it is not any cheaper than buying a hova bator. Only real time its better to make your own incubator is if you need room for many clutches, then building a fridge size incubator or large cooler is good.
Another thing to consider is Styrofoam is bad for developing leaks, so having water in the bottom of it is bound to result in a leak (I have bought Styrofoam coolers for camping and had them leak all over the place due to improperly formed foam or something). But really, any appropriately sized (to your needs) container, a heater, thermostat and a spot to put eggs (shelves to set the containers in) which holds humility well, will work for an incubator. Could make one out of a plastic storage bin, heat cord (less likely to get too hot, and can be controlled with a simple simmer switch or rheostat setup) a metal tub of water (ie aluminum loaf tray) and some plastic mesh setup as shelves, will work. Course a storage tray be better if its more insulated but you can buy a roll of aluminum insulating wrap, or line with the dense foam sheets used for insulation or packaging.
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PHLdyPayne