SIMH images to Zip Disks

I have a bootable 100MB RSX-11M image in SIMH that I'd like to blast onto a Zip disk and boot through my CQD-220. I'm running a build on SIMH that's essentially identical to my machine, so it should be straightforward, but when I try to boot my disk, I get a "not bootable" response from the ROM (the activity light on the Zip drive does flash momentarily, so I know something is going on, but it doesn't look like it's taking enough time to actually seek anything (and when I boot through the bootstrap provided by the CQD-220 ROM, I get a HALT at 000002, which tells me it's probably getting all zeroes). I know at least some of you boot your -11s off of Zip disks, probably with similar cards. What am I missing? As far as I can tell, the SIMH disk image format is a headerless binary blob, but it doesn't look like I'm getting to the point where that even matters yet. - Dave