teklektik said:Don't turn that into a science project - you just have a small gap.
Go to a Home Depot or Lowes and get a foot of PVC shower pan liner sheet off the roll.
showerPanLiner.png
The roll is 48" wide so you'll get a huge 1' x 4' piece for about $5.
This stuff is 40mil (thickish but much thinner than inner tube), very dense, and pretty incompressible. I use this to mount all manner of stuff on the bike. You can wrap a layer or two, or cut overlapping sized rectangles to put under a layer to get an odd shape fill. This stuff is amazingly rugged and abrasion-resistant -- my go-to stuff for mounting, shimming, insulating heavy battery items, etc.
The thing is the batteries have a combined weight of maybe 5kg. If I clout a rock a speed the forces will be significant given the downtube is only around 1mm thick and a don't want to ruin my nice mtb - hence the cogitation
I like the idea, unfortunately its pretty difficult to cut hold of an off cut online here in the UK (i've spent a few minutes searching ebay and amazon), i'll try and pop down to my local hardware shop tomorrow.