[Wavytone]

Hamish congratulations on finishing the f/6, should be a nice scope.

IMHO focal ratio around f/6-f/7 is ideal as the useful magnification from typical modest eyepieces can easily span a 10:1 range without resorting to Barlows, and the secondary is not so large as to degrade the image.

Best to buy a mirror - as Joe suggested buying one costs less than having one coated locally.

Only reason to make mirrors - or recoat them - is because what you have is both quite special and irreplaceable - such as the mirrors in my 9" Santel mak. This is a rare beast and the complete optics are 1/20 wave RMS.

FWIW the Edmund Astroscan was f4.2 and only suited to low-power - it was quite disappointing on planets or the moon which will be most novices first targets.

PS your idea of making a trackball is a good one, if you have a reasonable sphere of the right size to start with.