There appear to be two main methods for holding your hive boxes secured together: the strap + emlock or spring clips (also known as Z clips).
I decided to use the spring clips as I thought they looked neater and also rather cheap.
I got the spring clips from Hornsy Beekeeping and used some small screws I already had.
Whilst they are relatively easy to install, I wanted to find some documentation on how to do it, but it is quite lacking. The only decent details I found were from David Cushman's website which shows a variety of methods for a very similar looking Z clips.
After examining my boxes and bottom boards, I went with slightly different dimensions because of the way the bottom board for the flow hive is constructed (with the sloping pieces towards the back). Instead of 20mm from each side, I went with 25mm on the lower side and 15mm on the upper side. Also, the clips I had were reversed to the David Cushman version due to the way the "handle" is shaped, so the clips is screwed in on the right side, no the left.
In order to make it consistent, in case I moved boxes around in the future, I did the same with the brood box to super box too. Note: I am using a plastic queen excluder, so the dimensions I used are fine for this as it is only a few mm in height, but if you are using a metal excluder which I think is about 5mm (?), you may need to adjust for this.
I found that the clips appear to be quite strong when clipped in, and whilst quite simple, look like they should do their job.
Here are some more photos:
I hope this can help anyone who wants to also do this installation.