Silver solder/flux core. One of the best is Deans Brand. I buy the larger spools that can be found at HomeDepot or Shitshack.
MaxipadAmps has the a line of Lipo pack heat shrink also. Hate to say this but it's of excellent qaulity and I buy it myself. I have to have friends order it for me tho because somebody hates me over there and have canceled my orders refunded me and couldn't guess why ?
I didn't realize you needed the shrink for the exterior of the pack,I thought you were refering to the wire, sorry.
FYI, 24mm = 1 inch