The floating thin sections on layers, lack the flow consistency to maintain temperature regulation of the heat block. The heater cartridge in the print head is managed by a PID control loop. This will always have some overshoot and undershoot of temperature when the flow changes substantially. There are a number of contributing factors to this issue in the printer hardware design. I could go into a lot of depth here but that is an aside.

I would not use thin floating sections. Let’s say the whole backing was solid in a FreeCAD design body. I would then do something like an egg shaped ellipse pocket out of the middle. Another option might be a % like shape with the thinnest section offset so that the layer lines are still substantial.

My most advance approach would be to print the face of the frame on the bed, and the rear face of the backing plate also on the bed as a second part. If designed well from the start, and if the bed is large enough, you design the print to finish the backing plate before the face is completed. Then you add a print pause, remove the backing plate, insert it into the front plate, and continue the print to encapsulate it as a single part. This makes any 2D pattern for the backplate possible, and you do not need to deal with fasteners or whatnot. You end up with a perfect picture frame slot using this method.

A total aside, but this idea can also be used to make your own printed supports manually for perfect overhangs. You print the support to size, add a pause, and remove the printed support shape. Ideally, you add a ~0.1mm-0.2mm clearance gap, paying very close attention to how your slicer layers height and first layer correspond to the support dimensions, or rotate the support to utilize better x/y dimensional accuracy if possible in some designs. You can even create a printed alignment jig on the build plate just to hold this manually created print support. The trick is to then apply gluestick to the interface between the manually printed support and your overhang. This can produce nearly first layer like print quality on an overhang with dimensional accuracy too.

Another super advanced trick: let’s think if the picture frame standing vertically upright in Cartesian planes. It is facing forward on the X-Z plane (X = -><- = >< = left to right). Let’s assume the origin 0,0,0 is properly centered in the frame. Now if we look at a X/Y Top, section view, we are looking at the picture frame as if someone had used a hacksaw in the middle of the sides of the frame. In other words, we are looking at the frame’s profile view. Now typically, people approach this like a [. Now this takes a lot of practice, but it is possible to design a profile something like ɭ̅̅̅ ̅ ̅˻ ̅ ̷̅ – the print bed is ↓. If you design this just right, the left side is the frame and the right is designed with a small connection to the bed and an angle where this connection is close to the rest of the frame. The thin bridge overhang is going to contract and shrink towards the larger heat mass of the frame, especially because of the printed layers above the bridge. This contracting force will be set into the part like a spring, but will remain compressed due to bed adhesion. You may want to add a small first layer connection or inner skirt to hold this section in place firmly throughout the print. When the part is removed from the bed, the bridge spring will pull the right section back. This will create the slot for your picture and potentially a way of holding other types of backing, while not worrying about conforming to other types of manufacturing process constraints like a wood router or sheet metal profile.

Sorry if my lack of eloquence, verbosity, or tone come across negative at all. I wish to be encouraging and am just nerding out.