Thanks. Some developments which may have defined the path ahead to an extent.
After considering some limited opportunities for full range drivers here in Australia to my knowledge I decided on the Dayton audio PS180 nominally 6.5 inch diameter driver. Although showing as in stock there will be a delay in receiving it now so maybe it isn't.
Specs at the link
https://www.parts-express.com/pedocs/sp ... 0Sheet.pdf. For some reason the Dayton audio site is linked the parts express?
The box calculators tried have consensus of around 70-80 Hz in ported box of about 15 litres. I will try all the software I can find for this driver and the box size of 15L and look at the range of results. Not reaching the full 20khz may not be harmful as my recent hearing test shows steep rolloff around 8 kHz. Others listening with me, not often, may appreciate the full range though so I should do my best. As before if anyone knows a trustworthy box model I would be grateful. Also once the volume is calculated are there golden ratios for length width and height? I've heard of the golden ratio in photography based on nature etc so probably exists for audio as well. I have an idea what I would like the dimensions to be but would be good to model whether good bad etc.