I have just finished the job of lowering the engine/gearbox combination out of my new (to me) 997. Like you I searched for a suitable engine cradle for this car and finding none, decided to make one.
In short I used the adapter for the 911 3.0 litre engines in conjunction with good marine plywood braced with 2.0cm square tubing together with strategically placed uprights to produce a stable platform. I was somewhat lucky in my placement of the adapter as it proved to be in a near perfect balance position. There was no need to put anything under the gear box during the drop as the whole lot came down in a near horizontal situation.
There was quite a bit of trial and error involved in making the platform but the time proved to be time well spent as there was never a hint of instability during the drop.
The photo's tell the story.
