Help with large piece in narrow frame

Looking for some help on this one:
1. I have a client that has large (replaceable) photos, largest being approx 1700mmx1000mm (67"x40").
2. They want to use a frame that is 15mmx20mm (say 1/2"widex3/4"deep) they will not budge on this. It is too shallow for a strainer frame.
3. Acrylic glazing.
4. Photos to be mounted down - decorator pieces only, conservation is not an issue.

I'd prefer to mount the photos to F/core but I think I may need to mount it to 16mm mdf/ply to stop the frame from flexing. Has anyone done this? Would a 16mm backing be enough or can you suggest some other options?

TIA

 

Use an aluminum sign blank for the backing. A sign shop can cut it for you and then just screw it onto the back of the frame just shy of the edge. Use Wall Buddies for the hangers.

 

Jeff's suggestion is good.

Another way to approach it would be to mount to Mighty Core and fit into the frame. Then take fiberglass screening available from any hardware or big box store and staple a piece across the entire back of the frame. Fiberglass screen is dimensionally stable and won't stretch. It is light weight and inexpensive.
You can then put a dust cover right over the stapled screen and cut or sand off the excess screen and paper. I suggest Mighty Core because in that size foam core might warp enough to warp the frame and Mighty Core should stay flat.