Thanks, although really it's not the cap that's the issue...it's getting a CO2 cannister to fit which is the problem.
I suspect customs wouldn't be too keen to let those little CO2 bulbs through!
You're not limited to sparklet-type cylinders. I used to use a 350-ish g Hambleton Bard cylinder, about the same size as a Sodastream.
The simplest way to go is to get an adapter that connects a Sodastream bottle to a gas regulator. Some of the sponsors here will do you one for $39 plus postage, or look further afield. Now, instead of screwing the adapter into a regulator, get hold of a backnut and a couple of washers and fix it to your cap instead. The backnut thread could be a bit of hassle because the adapter doesn't look BSP to me from a casual inspection. This discussion:
http://www.aussiehomebrewer.com/forum/inde...2329&st=210 suggests it might be 5/8-18UNF.
Next problem is the pressure relief valve. I would seriously consider dispensing with it, as either the cap or the tap ought to blow long before the barrel deforms.
Lastly, as I suggested before, the tap is crying out for an upgrade if you've got the readies. A 3/4" metal ball valve would be the simplest and cheapest way to go here, plus backnut and washers of course. Optionally, get a threaded tap so that some sort of sparkler device can be improvised, like one of those fine mesh filters for kitchen taps. I guess the desirability of this feature depends upon whether you're a northern or southern Pom. B) A quick look at the sponsor's sites suggests they only do 1/2" ball valves, so that might require a trip to an irrigation specialist (provided you are happy to use nickel-plated brass). Plenty of folks do on their boilers, and I can't really see any additional problem with fermented beer.