I suspect your going to end up binning the idea, rather than tweak it.
Basically, you've created the world largest 'bum bag/fanny pack'. There is a good reason you have never seen such a thing in your local camping store - its a horrible idea. Your attaching a big bag (which in the case of the artic bag, weighs 2Kg, at the very least) to a bit of webbing, and putting that around your waist. Your going to knacker your back, be extremely uncomfortable after a short walk, and use lots more energy than you need.
The reason your doing this is because your using a surplus mountain pack, which gives you about 35 litres of storage. Thats bascially a day pack. You could do with a bag that size in summer, with your Jungle bag. In spring in the Cairngorms, thats not doable, unless you join the ultralight hiking types, but that means spending about 10 times what you paid for your arctic bag on a very high spec sleeping bag.
Ok - so your taking the arctic bag - good! Your mat is a bit basic, and that could do with an upgrade, but the biggest problem is that your trying to get a quart into a pintpot, and then coming up with a bad solution to carry extra kit. Buy a bigger rucksack. You dont have to spend a fortune, and you dont need something as big as your last one, but a 55-65 litre rucksack would make the world of difference. Go Outdoors dont have any real bargains at the moment (they had a LA bag for £68 a while back), but you could get a a Vango for less than £45, which probably would do for a while.
If you do want to go surplus, look at the Stings from Endicotts
http://www.endicotts.co.uk/componen...ck-dutch-l-a-sting-d-p-m-2464-detail?Itemid=0 , awesome build quality, and 55 litres without extra pouches. Grade 1 from £40. Or a German Army 65L rucksack - very basic waist belt, but from £24 (or even less on ebay)
http://www.goarmy.co.uk/army-bags/army-rucksack/germany-army-rucksack-65l-flecktarn-prod
- and you can use the folding mat you've already bought. You might even find something decent on Ebay - I spotted an old school Karrimor 65L sack for £30. And dont forget the sales on this site - someone might have something for the right price.
Make you life easy - get a bigger bag, which means everything can be carried inside it. More efficent, more comfortable and just much easier.