The G-Drive USB-C 1 TB portable external hard drive would fit the bill very nicely, then, and it is not terribly expensive. I think it is $89.00 at the Apple store. It also comes in the same colors as the rMB and the 2016 rMBP. I have mine in Space Grey to match my rMB! It is a very nice and pretty fast drive (I believe it is 7200 rpm). It is also flexible in that it comes with two cables, as I mentioned, so that if one has older Macs with USB 3.0 it's easy to swap around and use the drive with both types of ports.
That said, if it's major speed and real portability (as in very tiny and very lightweight) that someone is after, then the way to go is with the Samsung T3 SSD (or other brand, there are a couple; I am most familiar with the T1 and the T3, so can't say much about others since I have not used them). True, the external SSD drives are significantly more expensive than a platter/spinner hard drive, but in some situations that extra cost may be well worth it.