I'd either get a real counterpressure filler or got the DIY route... the beer gun seems expensive for something lacking true counter-pressure...
Also, regarding the DIY version: If you get the right sized tubing, you can put it through a drilled rubber stopper and stick it right in your draft spout. This eliminates the need for a picnic tap. This only works with the widest spout attachment on my ventmatics, though, so if you have regular rear-seal faucets it may not be possible... not really sure.