Help with a framing issue

I'm building some largish framed prints for a customer. They are 24X36 and 24X28. Long story short, I've got acrylite glazing, a 4 ply mat, 3/16 sheet of foam core and 1/8 sheet of foam core in a metal channel frame. It all fits, but I cannot use the spring clips in this situation. There is a slight bit of slack in the frame and I need something to take the place of the spring clips to take out that slack. Technically, I can't get the spring clip under the lip of the frame channel. There is a bit more room in the channel. Hopefully some of the framing folks will have a suggestion.

Thanks in advance,

Mel-

Mel, do not know how much slack you have, but why not try another piece of mounting board or mat or 2 in the back? Those clips to me really bite anyway!

Mel,
I usually use strips of mat board to fill small gaps. It slides in easier if the lead edge is beveled.

I usually use strips of mat board to fill small gaps. It slides in easier if the lead edge is beveled

I have made corners of mat board to slide into the corners and also strips like mentioned above. I use the leftover mat board from the center cutouts. They are corner shaped, beveled and ready to slide.

Thanks for the idea with the beveled mat board. I think that will work for this situation and I'll try it when I get home. I didn't want to just shove more stuff inside the frame.