Digging into the Seidio cases I see they state that the Surface cases add 2mm of thickness so I assume that means the Surface holster will be 2mm wider and taller.
Pixel 2 Dimensions: 145.7 x 69.7
Pixel OG Dimensions: 143.84 x 69.54 (adding 2 mm for the Surface = 145.84 x. 71.54) This is very close
Galaxy S6 Dimensions: 143.4 x .70.5 (adding 2 mm for the Surface = 145.4 x 72.5) Also very close
I used a S6 Clip Holster for my OG Pixel and it was a perfect fit. Assuming the Surface adds 2 mm in width and height to the holster then either of the above *might* work. In this case I'd say the OG Pixel Surface Holster would be the better bet.
I am going to hunt for a good deal on one and if I find it I'll snag one and report back..