Low pressure commercial regs are pretty cheap. If you find one locally or on a surplus site, you could try to meet its input pressure, usually 150-200 psi. I never had much luck with using an AGD flatline bottle at low pressure on a marker, but it might work to feed another reg with a relief valve.
Seems like you only get a few minutes out of a 68 cu in bottle though. I think we've done this calculation before.
Last edited by Spider!; 01-29-2013 at 05:41 PM.