Looking for drive enclosure to boot PowerMac

The internal drive architecture in my PowerMac seems to have screwed up to the extent that even the tech folk in the Apple Store can't pinpoint the problem. And even if they could, I'm told, Apple (par for the course) don't support these machines any more, can't get the parts etc etc.

So I'm trying to find a Firewire enclosure I can put my SATA drive in, to use it as the system drive.

This is proving harder than I had thought. I've already ascertained that (contrary to Apple Store advice) the Mac I have can't boot from a USB drive - and now I've just returned one "Mac-compatible" FW enclosure that I can't boot from either.

Can someone recommend something that works?

( I'm based in the UK... )