OWC makes a few good enclosures. I use a Thunderbolt2 enclosure with 2 drive slots, one with an 3.5" SSHD boot drive, the second a 7200RPM large drive which is used for external storage.
Speeds <> the enclosure are:
- Thunderbolt 3 (USB-C type connector) ~40Gbps
- Thunderbolt 2 (MiniDVI type connector) ~20Gbps
- Thunderbolt 1 (Mini DVI type connector) ~10Gbps
- USB 3.1 ~10Gbps
- USB 3 ~5Gbps
- Firewire 800 ~800Mbps
- USB 2 ~480Mbps
- Firewire 400 ~400Mbps
- USB 1 ~12Mbps
Check the specs on your Mac to find out what ports it has, then choose the fastest that is compatible with your Mac.
Generally, anything under USB3 is not a good candidate for an external boot drive. SSD\SATA3 drive transfer speeds are ~6Gbps. The additional bandwidth might be useful if you attach other devices in the chain. Thunderbolt can daisy chain displays, or other Thunderbolt devices, so the drive may only use 6Gbps of bandwidth but the rest could be used to drive displays or other things.