Finding capacitors with low soakage (dielectric absorption)
The dielectrics with the lowest absorption are vacuum and air (which is jinxed in normal conditions by porcelain, glass or sapphire). Out of normal dielectrics, Teflon was the best in the last century. There have possibly been some improvements made for polypropylene and polyester caps. There is an article by Bob Pease about the phenomena http://electronicdesign.com/article/analog-and-mixed-signal/what-s-all-this-soakage-stuff-anyhow-6096
So your solution may involve Teflon or normal plastics. But if you are designing a new 9 digit voltmeter, then the capacitor design becomes very interesting, as interesting as a real analog trade secret can be.
BTW, the percentages for DA cited on Analog Devices pages are in the range of 0.001%...