The clamps you have are designed for an oversised section of the base bar. Unless Deda also make a clamp to suit the narrower section (unlikely) or spacers to suit (also unlikely), you MUST mount the clamps at the place they are designed for. Custom designing a shim usually requires skill and engineering tolerances beyond most home or shop mechanics and could lead to damage of the handlebar at the very point on the bar where stresses are highest.
If you're stuck with this type of clip-on the best thing to do is to mount the clamps where they are designed to go and then modify the position of the elbow-pads and the extensions to provide the width you need. Brackets to widen the pads are much easier to make (using steel plate or something like it), and most clip on bars are simply 1" OD tubes (it pays to check though), allowing you to mix and match extensions from different brands to get the required hand position.
Alternatively you could hunt for a new type of clip-on.
Most aero bars are not particularly flexible for getting the position you want. Even pro teams need to make modifications (sometimes big ones) to get into the right position. I prefer my hands to be lower than my elbows, but found that my bars (Profile T2+) had elbow pads too low compared to the extension height to be comfortable, so I made some chocks with longer screws to raise them. They're now close to perfect.