TL;DR: Don't. Get an SD card.
Let's start by taking a tour of your MacBook's I/O:
For sake of simplicity and feasibility, we'll explore three options here: USB, Thunderbolt, and SD.
Contrary to your original question, you do have USB 3.0! We won't go into it much, but the newer port you're referring to is USB 3.1 Type C.
USB 3.0 has a maximum speed of 5 Gb/s–below SATA 3's 6 GB/s, but still above most (even fairly fast) SSDs. All good then, right?
Unfortunately, through a combination of packet problems, controller issues (either on the computer or enclosure end), and general magic, USB 3.0 just... isn't that fast. I'm not particularly well equipped to say why, and I wouldn't be surprised if more in-depth analysis revealed flawed testing methodology, but that's the fact of the matter... for now. Still, 3.360 Gb/s isn't anything to scoff at, nor is it too far from advertised.
Thunderbolt is kinda the big daddy of external expansion. It advertises 20 Gb/s over a 4x PCI-e 2.0 connection. And for the most part, it delivers... 11 Gb/s. Akito specifically cites the chipset (rather than the protocol) as the limiting factor. Still, that's enough to handle the fastest SATA SSDs, and even decent PCI-e drives.
The SD spec promises speed up to 2500 Mb/s for UHS-II. You won't get that on a MacBook Pro, since the reader is connected over a USB bus–in your case, USB 3.
One thing to note is that the maximum file size you can put on an SD card varies. Normal SD cards support 2GB (avoid!), SDHC cards support 32GB, and SDXC cards support 2TB.
Choosing your connection
With this in mind, we get the following:
- USB enclosure: Lower speed, greater flexibility with drive choice, medium price
- Thunderbolt enclosure: Better speed, greater flexibility with drive choice, PCI-e support, high price
- SD: Lower speed, lower flexibility with drive choice, low cost, much better portability
Note that USB and Thunderbolt are fundamentally incompatible specifications; you cannot convert Thunderbolt to USB.
Given that you're using this for secondary storage on a computer with an already very fast SSD and (based on the fact that you said $100-150 was too expensive) price is important, SD cards seem like the way to go. You can even get ones that lie flush with the rest of your laptop, or microSD adapters that allow you to bring your own SD card.
Since you haven't provided a specific price or storage requirement, I can't get more specific than that.
If you have no regard for money or physical space considerations, the Sony 128GB SF-G Series UHS-II is among the best I know of, and comes with a 5 year warranty and data recovery to boot, though I would advise against this. I would recommend something along the lines of the SanDisk 128GB Extreme UHS-I and one of those microSD flush-fitting adapters from earlier.
The exception to this is if you want a large amount of storage (256GB+ mostly.) SD cards at this price point are still much more expensive than their desktop counterparts while offering worse performance, though this breaking point will likely change in the future as SSD prices continue to drop.