There are quite a few around now that will allow you to install a PCI-E m.2 SSD, prices from about £30+ . Most of them I believe use a JMicron controller to USB 3.1 Gen1 and are limited in bandwidth, although there was supposed to be a 3.1 Gen version due out last month.
If you buy one, make sure it's a full mental enclosure and comes with thermal pads as it will get very hot and likely throttle causing even worse performance than the interface it is being connected by.