Acessible- yes (down the hill from the Loch Lomond road & straight across the traffic lights), Free - Not so sure..... It used to be, but I think I saw a pay & display meter last time. I would imagine charges stop at 6, and it might even be "April to October", but I doubt it......
I am also giving a thought to a slip on the Gareloch near Blairvadach Outdoor centre (not the centre's slip, unfortunately
) . It is also easily accessible, and is free but WILL need some illumination to assist in getting back out, and if the tide is down, will need a rope too. (but you'd need a rope at the pier anyway at low tide)
The Blairvadach one is mentioned on Boatlaunch, and It's not quite as bad as the description suggests - I've launched & recovered both the SR4 and the Humber there with no issues, although it does get tight when the lazy people who leave their cars & trailers right by the top of the slip don't think that someone else might want to use it too.... But that shouldn't be a problem at the end of November!
Personally I'd prefer the Blairvadach one, as the pier I am told is shopping trolley central at low tide, and has next to no ladders at that side. If there's two (maybe more?) of us that could work. I'd think you could get a Destroyer down there, A Pac22 would be a squeeze tho'.....
Would it be worth starting earlier & seeing the upper reaches in dayight? (or has everyone done that already?)
On the subject of which, I visited Halfords last night. Gave up on the front fog light idea - much as 2x 55W lamps would have provided ample illumnation, my car's lighting fuse would have taken offence to it, so I am now the proud owner of two reversing lamps - £6 each, half the size and a third of the weight, and no worse power consumption than the trailer lighting board!