You an get face frame clips, and rear suppot clips for most any full extension drawer guides. Looking at the pictures, looks like you are loosing about an inch and a half on each side of the drawer boxes. By building new drawer boxes, you can gain back a couple inches of width. At the rear, you could run a single vertical of ply on each side to fasten guides to, along with face frame clips at the front. Remember, these are drawers/ trays, not furniture, so butt joints, glued, with pocket screws are more than adequate. You can make both sides and bottom from 1/2" Baltic birch ply, or the 1/2 " birch plywood (Columbia Forest Products) that HD sells. Cut a rabbet on sides and front of bottom, with a dado in sides and front. Rear just drops down onto bottom with glue and brads. Cut some edging for tops which are visible.