Although it’s not a common home improvement project, occasionally it may be necessary to change the size of the ceiling hole that holds a light fixture. By far the most common example of this is when replacing an older recessed light fixture with a more modern style. Older recessed light fixtures (also called canister lights or can lights) typically used a fairly large housing that required a large ceiling hole. But modern canister lights—especially those that use newer LED light bulbs—are often quite small. Trying to install these mini canister lights can be difficult when the hole for the old fixture is too large.
There are other instances when changing the size of the ceiling hole may be necessary. For example, you might want to replace the recessed light fixture altogether with a standard flush-mounted light fixture that requires a much smaller ceiling hole. Or perhaps you’ve made a mistake when installing a light fixture and cut a hole that is too big for the new light. Another possibility: You’re moving a light fixture to a new location and need to cover the old hole completely.
Before You Begin
The project described below is a home improvement “hack” that makes use of a device more often found in the kitchens of professional bakers—a cake separator. A cake separator is a rigid polystyrene disk that bakers use to support and separate the upper tiers of tall, multi-layer cakes. Because of its 8-inch diameter, it is the perfect size for reducing a ceiling hole to accommodate both 6-inch and 5-inch recessed lights. Its smooth, rounded face and edges look like actual trim. Because it is made of hard polystyrene, a cake separator will be rigid in form and have good heat resistance yet the material is still soft enough to cut.
If you can’t find a cake separator, any thin, rigid plastic disk can serve the same function, provided it is strong enough to hold the light snug up against the ceiling drywall, soft enough to cut without damaging it, and thin enough that the recessed light clips will snap down. It should also be attractive, aesthetically in line with the rest of the light, and suitable for painting.
Source: The Spruce