Lots of options! You could use an IDE drive (just snake a cable outside the case), or a slim drive mounted internally (lots of people here do that, if you're not scared to hack your case up a bit... one guy even has one mounted inside an A600 - there's pics here in the forums). Or since you already have SCSI you could use an external drive in a chassis, if desk space isn't a constraint for you. That's what I do:
Edit - just realized you said you had a Squirrel, I was thinking something else. There's still options for internal SCSI modules though, like this one:
http://amiga.resource.cx/exp/blizzardscsi4
