Beverage dispensing from compressed 2 liter bottles (question)

Here's a brief overview of our current setup (for a school project). We have a CO2 tank with regulator connected to 6 2-liter bottles. Currently we have drilled two holes in the top of the 2-liter caps, one for air in, and another for liquid out. We are controlling the flow out by electric solenoid. However, the problem that we've ran into is that we can't securely attach the tubes into the caps. We have tried silicone (aquarium sealant) as well as super glue and caulking (desperation attempt), but nothing is really working in a reliable fashion.

Does anyone have any suggestions/ideas that would make this process much easier for us? We were thinking in getting rubber stoppers from chemistry application (that have 2 holes in them) but the problem is we have 2 different sized tubes and the holes in these stoppers are normally very small.

Sorry if this is in the wrong place or if this is a question that has been answered; I've been searching everywhere and have yet to find a solution yet. Thanks.

Maybe try a non drilled stopper and drill the size hole you want in them?? Just a thought. No practical working knowledge on how good/bad this would work out.

Does drilling through rubber work fine with regular drill bits? I've never done it and I would just be worried about ripping them apart if I tried haha

The drilled rubber stopper could certainly work.

You could also try drilling through the cap, then using a bulkhead fitting with an o-ring. I'm not sure what size barb you're looking for, but maybe something like this:

http://www.mcmaster.com/#barbed-through-wall-tube-fittings/=mip598

The problem is that they need a pretty large opening, so you're going to run out of room in the lid pretty quickly.

If done right, epoxy should make a relatively gastight seal.

Drilled stopper would work but you may need to drill a large hole in the stock cap to use as a retaining collar if the pressure is too high. You can drill a stopper by putting a piece of metal tubing in a drill. Filing a few notches in the end of the tube works faster.

The pressure we're using is relatively low so I don't think too much pressure would be an issue. Thanks for all of the feedback though; I'll look into the stopper idea for the time being as that seems to be the cheapest and simplest.