I also have the Tuareg Tank bag and have cut the base padding slightly to accommodate the filler. But I have cut off the original fittings to secure the bag as you have to release the front one every time you want to fill up and you have to completely unzip the base from the bag leaving the base on the bike when you leave it. This is very awkward when the bag is full and even more difficult when it's raining and cold and dark (We've all been there!) So I purchased some quick release fittings, 80 pence each from my local boat chandlery, and cut off the offending parts and secured these clips. I superglued the respective parts on with the intention of stitching as well but the glue seems quite substantial. The side panniers are some small ones that I got from the same supplier and used all sorts of straps to support them. They fit neatly in front of my knees and can carry a variety of the rubbish that normally follows me around. The smaller bungee material is there simply to support all of the harnesses and keep them roughly in position when I unclip all of the bits. The whole set up works very well and the tank bag costs about �40, sidebags �20 each so all that lot for less than �100, result. On one note the zips on the tank bag are not that substantial and the really important one from the bag to the base broke the puller tag. I complained to the shop and they said that they would send it away and get a new one put in. That's all fine but a) I'm now using my bike and bag, and b) I have secured my controller for the heated jacket into one of the sided pockets on the tankbag and that is going no-where! So I told the shop to shove that idea, I think that's why he gave me a bit of a deal on the side bags!
PS the sidebags are just small bags that you can secure anywhere, not just where I have them.
Hope this helps
PPS Sorry about the glorious weather in the pics, and the fantastic campsite, Tibbie Shiels Inn, St Mary's Loch (Borders) beautiful weather all weekend, did about 550 miles in all!