In a similar position with aero bars that do not allow independent adjustment of extension tilt (without tilting the base bars, which I don't want to do). I'm going to use J bend extensions fitted in reverse (Use Tula) to get the hand position I like. I haven't done it yet, but my plan is to modify my Cee Gees Cushy pads to give me a little bit of a ramp. They are really thick, if I remove some material from the end nearest the rider I should be able to get a bit of tilt. The tricky bit will be bonding velcro hooks to the underside so it still sticks to the arm rests.

But that's an idea. Depends how much tilt you want. Problem with angled spacers is that the bolt heads will be angled from the countersink in the upper surface of the rests, it might not line up very well. You might also need an angled washer on the top side of the rests, which will protrude.

Picture here shows how much material there is to play with. Not very much, admittedly.

Potentially a spherical washer?

Why don't you just angle the entire basebar contraption upwards. First of all having the basebar angled upwards will be more safe in an emergency (especially on a downhill) since your hands won't have the tendency to fly off the front, and you can get the exact tilt you want in a span of 30 seconds. The aerodynamic penalty of base bar angling should be 95% overcome by a better rider position. Seems like a very worthwhile tradeoff rather than mucking around with pad modification which more than likely won't work that well. And maybe I am a bit biased against horizontal base extensions....seen too many bad things with people losing control in an emergency. I don't have to see that many crashes or close calls to know that there is an inherent lack of safety in the emergency mode....maybe its my background from the first career in the aerospace industry where we designed systems first as safety critical then as mission critical. I'll turn off that rant though, and just focus on the potential aero gains....try angling it upwards and do some field testing. Worst case you angle it back down and no cost, nothing permanently modified.

I hear you Dev, but I am also an engineer and I quite enjoy tinkering. These are the UCI-illegal Devox bars with the particularly deep aspect ratio, and I have read and listened to the wise words of Jim Manton and Andy at Alphamantis amongst others, and I'm aware there is little penalty to a point by tilting the base bars. My preferred hand position is pretty high. At the end of the day I'm a tinkerer and I enjoy sweating the small stuff, enough to take a drill to a very nice carbon frame so that I can tuck the cables away. It is almost a hobby in itself..!

But I agree, not at the expense of safety. I used to have the Vision Trimax bars with the upturned bull horns and I did notice the difference when I swapped over to these Devox bars. That isn't the latest photo - I have put a couple of extra turns of bar tape near to the levers which gives a little bit more to brace against. I've thought about picking up the Felt grips that go over the bull horns, that would be ideal but not easy to source over here in the UK. But with what I have I've ridden all sorts of terrain in all sorts of weather - I'm in West Yorkshire. I am happy with the safety factors.

I took 2 of my extension spacers to a local machine shop and asked them to bevel a 10 degree angle onto them, and then used spherical washers under the bolt heads. Torqued to spec and seems solid.

So your stack looks like
(top to bottom)
Spherical Washer
Extension clamp
Riser (with bevel at the top)
Basebar


Now to find a machine shop in NYC...

Integrated P5 stem/bar