I figured out how to create these transitions in Shotcut:
How I did it:
I came across Transquare here:
It was originally devised for Vegas.
I downloaded the free pack, which included two .mov files: a MASK file and a SHADOW file.
I took these two files, loaded into SC and exported them as MP4 files.
I then made 3 variants of these files - 1) slowing them down to two thirds speed, 2) slowing them down to half speed, 3) rotating by 90 degrees (I had to make them larger to fit the screen).
So - I ended up with 8 MP4 files, a MASK and a SHADOW version of the original and each of the 3 variants.
So - how to apply to clips in Shotcut.
For the first one:
Put 2 clips on V1.
Import one of the SHADOW MP4 files and place on V2. Line up the end of this MP4 with the end of Clip 1.
Slide Clip 2 backwards over Clip 1 to create a transition. Slide it right across to the position marked by the beginning of the Shadow file.
Select the transition, select Properties, Custom, then point to the corresponding MASK MP4 file.
Select the SHADOW clip on V2, apply a BLEND filter and select “Multiply”.
Ta - Da - a nice cube effect.
My demo features all 4 variants - 1) original, 2) two-thirds speed, 3) half speed, 4) rotated.