Having tried and failed at the @DocAnna method (my fault, not hers) and being chastened by @Coffin Dodger for not doing my research properly I decided to create another option for pimping a foil balloon to collect CO2 (yes, not enough to do).
So, take your balloon and using a pair of scissors cut the end off where the restriction starts.
Open the throat of the balloon..
...and gently create a cuff by turning the edge over on itself (a bit fiddly).
Take your choice of tubing, I use 6mm OD pneumatic hose, and apply a generous bead of silicone sealant around tubing about 25mm from the end to be inserted into the balloon..
Carefully insert the tube into the balloon opening and squidge (technical term) the sealant around the inside surface..
Now the messy bit. Get a small cable tie and loop it together. Then flatten the balloon opening against the tube (sealant should ooze out) and wrap it around on itself. Slip the cable tie over the join and tighten to hold everything in place. Smooth the sealant around the tube/balloon joint..
When sealant has set, trim cable tie and fit tap (if required).
So, take your balloon and using a pair of scissors cut the end off where the restriction starts.
Open the throat of the balloon..
...and gently create a cuff by turning the edge over on itself (a bit fiddly).
Take your choice of tubing, I use 6mm OD pneumatic hose, and apply a generous bead of silicone sealant around tubing about 25mm from the end to be inserted into the balloon..
Carefully insert the tube into the balloon opening and squidge (technical term) the sealant around the inside surface..
Now the messy bit. Get a small cable tie and loop it together. Then flatten the balloon opening against the tube (sealant should ooze out) and wrap it around on itself. Slip the cable tie over the join and tighten to hold everything in place. Smooth the sealant around the tube/balloon joint..
When sealant has set, trim cable tie and fit tap (if required).