Choose one of those. A mini IDE solid state disk or CF-IDE adapter combined with a 2,5" IDE-SCSI bridge should do well. 1,8" to 2,5" IDE adapters are also available. Even a CF-SCSI adapter in 2,5" hdd form factor is available at times.
edit: today I tested a Stratos CF PowerMonster for the first time. Now a Color Classic happily boots from a tiny CF card
Results are provided in the wiki page Flash Drive Test Results.
2nd edit: now I tried that same CF PowerMonster to boot a Power Book 180 and it works great. Even the very old CF (32 MB) card I used in combination with the SCSI-CF bridge reads much faster than the 500 MB HDD (which usually lives inside my PB180).