Stem not gripping handle bar tightly

Is this Mother T an ex-SNetter? 

Best shim is an aluminum soda can.  You can cut one with metal shears or heavy-duty scissors.  Make the cuts radially that the shim is curved.

Sandpaper (as mentioned above) is also a good solution.

Any chance you have a 31.7mm bar with a 31.8mm Stem?

I know Deda are normally 31.7mm...not sure if that tiny 0.1mm makes that much difference...

Carbon assembly paste is always a good idea, even if not carbon surfaces for both stem/bar.

Also...Tighten the top faceplate’s clamping bolts until the top is completely closed and the faceplates sit flush against the stem body. This ensures that the clamping forces will be even across the bar (then check that the torque is correct usually 5-6nm). If you have far exceeded this, there is a chance you may already have deformed one or other of the components and that is why they may not be seating properly.

use toothpaste or some grippy compound

and get another frame.. 90mm is faaaaar to short.

at least you should be on 120mm. I only ride 130 and longer  

Have you re-checked the bolt torque after the first ride?   Repeat until the bolts don't tighten any further.

Fine grit sand paper folded to give double sided rough. You can glue the inside to stop slippage

If you're sure the stem is the correct unit to cope with your handlebar diameter, try some carbon assembly paste.

I don't know if this would work foryour bars, but I use sections of old inner tubes to make up width if I need to for things like lights. I find they work better than the spacers that come as standard as they are much grippier (grippy-er?).