Dave -

Sorry I didn't respond to your email earlier, I was out at the US
Nationals... I bought an aluminum "M" sized bottle from Wings &
Wheels and it works in my DG-300 just fine! My DG-300 is an early
serial-number and the "pocket" for the O2 tank is just under 5.5" in
diameter. Later models may have openings for the smaller "D" or "E"
cylinders so I cannot comment on all DG-300/303 models for sure. But
I really like the "M" cylinder I got (5.25" diameter). Combined with
the Mountain High EDS O2D1, it makes for a great setup!

--Noel