Our standard frames are about 9mm deep. So a typical frame with perspex, matboard and backboard will take up about 5-6mm and the rubber gaskets are used as packers placed at the back of the frame to take out any movement. Because the frames are supplied as a kit I can’t be sure what you are framing, and I supply a standard size packer. If they are too narrow or too wide you may need to use card, or I can supply foamboard offcuts that work well. Usually the packers we supply work well. I did have a larger one for a while, but customers were finding them generally to tight to fit. If you have ordered a frame without a matboard (i.e. perspex and backboard only) you may want to just cut up some business cards to place under the packers if the fit needs to be a little tighter.