I had previously used a Berghaus Freeflow 35(+15) litre rucksacks, which was good. When I was out shopping for a new one last year, I stumbled across a Vango Contour 60(+10) pack that appeared to be tailor-made for the SOTA activator. Plenty of pockets/compartments, and a drawstring system by which the main compartment could be one large, or 2/3 sections, still with one big one. Down the sides are loops and base pockets perfect for poles and antennas.
The good thing about proper rucksacks, is that the large ones can still be packed down fairly small if used sections are compressed. So this is the pack I use for all outings, from Tuesday night summer contest activations (radio kit, coat, drink) to a multiband backpacking overnighter (NW-012 August 2011).
I seem to recall it was very reasonable - significantly cheaper than the “main brand” rucksacks, but with the best set of features for me and my activating. I am pleased with mine.
And by the way, never mind using a rucksack as leg warmers. Get yourself a 4 man bothy bag - it’s like having an instantly available indoor waterproof heated shack on the hill!