Pine boxing, available in many sizes and in depths of 34mm, 46mm, 70mm and 92mm.  

The sizing of the boxing is the 'outer' measurements of the pine box, not the 'inner' measurements (so they are the same size as the glass/acrylic size in a frame). 

These pine boxes can be bought to make an existing frame deeper, or just to use on their own to decorate and fill with items, or make a moss living wall for example (you just need to add a backing card to affix your items to and a hanger at the back if you want to hang your display on the wall).  If your frame rebate allows, there should be room for your existing frame's glass, mount and then a couple of mm at least for the boxing to slot in to.  

Fixings can be bought separately from here if needed:  Fixings for Box Frame

The pine box width is 11mm - hence if you are wanting to know the inner measurements of the box, please take off 22mm from the width x length measurement for an idea of inner box size.

Pine is a natural wood and will often have knots within the woodgrain.

Want to know the mm (millimetre) measurement in inches?
34mm / 1.34"
46mm / 1.81"
70mm / 2.75"
92mm / 3.6"
(11mm width of wood is 0.43".  22mm total is 0.87")

Available sizes (remember, these measurements are for the 'outer' part of the pine box, not the inner, you need to choose based on the glass/acrylic size of your frame):

5.5x6.5" (our small)
6x6"
6x8"
6x10"
7x7"
7x9"
8x8"
8x10"
8x18"
9x9"
9x12" (our medium)
9x15" (our large)
9x20" (our xl)
10x10"
10x12"
10x14"
10x26"
11x11"
11x14"
11x15"
11x22"
12x12"
12x15"
12x16"
13x13"
13x35" (our xxl)
14x14"
15x15"
16x16"
16x20"
17x17"
18x18"
19x19"
20x20"
21x21"
22x22"
20x60cm (love)
40x50cm
40x75cm
50x75cm