This type of heater fits directly under the nestbox "IN" the pan below.If you have cages that don't use drop trays I bend wire hangers to hold the heater in place.Also please make sure all cords are out of the rabbits reach and where they won't get wet.My husband builds mine from any scrape lumber he has around,or you can use the 1" pine shelving from the lumber yard.You don't have to bother with a bottom board if your using it in a tray,but I line the bottom with heavy aluminum foil to reflect the heat back up.I raise Netherland Dwarfs so the frame size of mine are 8"wide,12" long,and 3"high.Size can be adjusted to fit your nestbox size.We normally buy the clamp on type utility light when there on sale and he use the the socket and cord off these.A "U" shaped groove is made in the center of one end for the cord to stick out,and a small block of wood is added to the same end to attach the socket too. Always make sure the bulb is within the 3" frame not sticking up. I use them if its going down below 40 degrees, making sure to turn it off its going to get hotter-you can cook the kits if it gets hot!!