Does anyone have any tricks for purging the back side of a full penetration fillet weld?

I'm going to be welding some 330 stainless and from the research I've done, the fillets should be full penetration and welded on both sides if possible to resist cracking during heat cycling. 

I am considering using aluminum foil and tape to seal off the backside and purge it (the size of the project makes it less ideal to purge the whole thing) but if anyone has a better idea I'm all ears.