I’ve long thought that the TF ashtray (even the shiney Ex 1966 Jaguar one ) was literally a waste of space, so decided to replace it with something more useful.
And cut myself a hole using the box as a template,mine was 7cm wide by 13.5cm long, my heater pipes are lagged, without the lagging you could probably go larger, but I wanted to keep the heat out of the box, so I only made it 7cm deep,