I have a totally transparent Kaweco Sport fountain pen and it is obvious that there is wasted room in the barrel beyond the end of the short international cartridge. I know I can modify a cartridge with a sac to make my own converter or I could use the pen as an eyedropper. The fact is that I like using cartridges and refilling them. Simply, I want to hack 2 cartridges to make a longer 'just fit' cartridge. I've already done that, but am looking for a suggestion to glue it back together. I thought MEK or acetone would work to solvent weld it, but it did nothing. Does anybody know what type of plastic the cartridges are made from and therefore the preferred adhesive to get a secure leak-free bond? Thanks