You don't say whether you want the converters to feed the ADAT inputs of the HD24 or if you are jumping ship and going to an audio interface connected to a portable computer of some sort.
In good-quality external converter boxes, there isn't a huge amount of choice for 16 channels in 1U height. In addition to the ones you mentioned, the Lynx Aurora 16 is a reasonable unit and has modular interface adaptors, so you could get it with ADAT and migrate to USB (or some other interface) at a later date without ditching the converters.
Have you considered using an EC-2 kit to upgrade your HD24 to an HD24XR? The converters in the XR are every bit as good as those in most of the upper-end interface units, and produce noticible a improvement over the standard HD24 at 44.1/48KHz in addition to the ability to record at 88.2/96KHz when required. An EC-2 kit is, unfortunately, not easy to find these days, as it (and the HD24XR) was discontinued a couple of years ago, despite still being shown as current on the Alesis website. It requires zero additional rack height as it replaces the converter boards in the HD24, and you get 24 channels.