Most of the ready-baked bootable OpenBSD images that I'm familiar with are (or were) geared towards embedded single board computers such as the Soekris and Alix devices. The primary objective was to pare down the operating system to fit onto smaller flash cards.
These days, now that flash storage devices have gotten larger, most people just install the entire OpenBSD operating system directly onto the flash media.
I can think of two main installation options for your situation:
- PXE-boot network install.
- Direct install to the flash/SSD drive from another computer.
The least complicated by far is the direct install method.
Here's what I would suggest. Connect the SSD to an internet connected computer. Boot that computer with the OpenBSD installer cd-rom. Install OpenBSD onto the recently connected SSD drive.
After the installation, modify the /etc/fstab file as necessary. You can do this manually at the end of the install before rebooting. Or you can boot to the install disk again and select shell, mount the newly installed filesystem, and modify with the sed command. Usually this means changing all the /dev/wd1x entries to /dev/wd0x for example.
Remove the SSD drive and insert into the destination machine. Try to boot it up. If there are problems booting, it's usually because the /etc/fstab entries are incorrect. Check the dmesg output as the kernel boots to see what the destination machines hard drive devices are detected as. They should be either sd0 or wd0.
Good luck!