Hi Ed,
If you want to go down the GWR Toad route I have all the Swindon drawings plus my late Uncle Roy Penny got a Gold medal for his own example at one of the ME exhibitions in 1990/1991.
The late Ron Gage of the Chichester club used a Toad as a driving trolley behind his 14XX GWR 'Dart'. My Uncle Roy did not use his for this purpose. I suggest sitting on a Toad for any length of time would become rather uncomfortable.
Most clubs have a standard design they use. If you are not a member of a club I suggest you join one!
Cheers,
Julian