Regardless of the painting surface you choose, the most important part of building a custom floater frame is measurement.
Make sure your canvas is in square and is not bowed in the middle. The width and height and depth of the painting should be measured at the corners. Depending on the weight of the canvas/muslin, if your canvas is wrapped around to the back of the strainer stock, an 1/8” can easily be added to the size of your painting. You should also look at your painting on edge, if you paint in a heavy impasto or built up encaustic, you will need to measure the highest point on the canvas surface and add that to the total depth of your painting.
The depth of the painting should be accurate because the floater frame is made to be approximately 1/8” above the artwork and acts as protection for the painting surface.
Paintings on prepared panels are measured the same way. If you purchase pre-made cradled panels like the Elephant panels from Artist and Craftsmen Supply the dimensions should be checked. I’ve found the sizes to be off by an 1/8.” Accurate measurements are necessary especially if you would like a narrow float in your frame.
Artists who use uncradled prepared panels can either have their work mounted to support frames/cradles or float their work in shallow floater frames.