If it is foam core board, it squishes down a bit. With other backing board, I drive brads through the board and into the wood. I hope this is not a frowned upon technique! There is also some stuff called Rabbetspace: http://www.framingsupplies.com/FrameTek/FrameTek.htm
I run into this problem often because I think some prints, drawings and watercolors do nice in narrow moldings that often don't give enough space for even a double mat.
I cut a bevel on the foam core so the points will angle down into the wood. Put the package into the frame and while it is face down, put a weight in the middle of the package. Then take an exacto knife and slice a bevel into the edge of the foam core.
You can also just compress the edge of the foam core. This will give you enough room to put points in.
I use the infinity hangers. They are the most versatile hardware, if you need to (and you don't) you can bend them to make them into any size offset and they work great with thin mouldings. I cover the screw heads with some artist tape before I put the backing paper on.
The double-pointed framer's points from Fletcher can be shot in at a severe angle and bent back over the protruding package. It only works for about 1/8" or so, but it's a great time-saver and does not scrunch the package if done right.
Hi DonB, When this happens to me I cut down some spare timber to 1/4" x 1/4" (or as required) and fix it to the back of the frame with glue & pins/staples to extend the rebate then fix in the normal way. Hope this helps
The Rabbetspace has the obvious advantage of even
pressure, but a bent point is only putting pressure on the edge of the mat package, in an
area in which should be well beyond the margins
of the sheet in the mat; so that should not cause
any problems. The one closing device that can
be seen to have provided uneven pressure is the
spring clip that is used with sectional frames.
The backing board, between some clips can be
found to have warped away from the glazing over time and strips of scrap board pressed into the hardware channel of the sectional frame will give more even support than such clips will.