Worlds easiest mod. Take one T-slot plate and bolt to cross-slide. Easy.
The T-slot plate was found on the junk table at Chronos - £4 (I love the junk table at Chronos). I'm not sure what machine it was intended for - possibly an Emco part?
The plate has four mounting holes sized for M5 SHCS. I drilled a pattern of six holes so that the plate can be mounted in either one of two positions. (That is why the saddle locking clamp is milled away on the top surface - when the T-slot plate is mounted in the forward position the bolt head for the saddle lock is underneath the T-slot plate).
When the vertical slide is mounted on the T-slot plate in the rear position, the cutting point will be close to or within the footprint of the saddle, and this arrangement maximises rigidity. The "Z" shaped plate available from RDGTools that is designed to mount the vertical slide off the tool-post is hopeless; it behaves like a mass on a spring and vibrates like a violin spring.
I have seen a home-made cross-slide that is wider than the standard item and has T-slots machined in; it is very professional looking, a nice piece of work. My solution isn't nearly so good looking, but it's cheap, quick and simple.