Since you can relocate the big capacitor and bend the regulators to the side, the highest components are the midijacks. I measured 21,4mm combined with the pcb.
If you align the boards the normal way (jacks to the back), you could solder the ribboncables directly to the SEQ4L-pcb. Otherwise the connectors would sit above the midijacks and add to the overall height.
am building a “slanted” version of it, the minimum space you need is 14mm above J15A and 30mm for the MIDI jacks (that is because the SEQV4L connectors are directly above them).
As Imp wrote, if you solder directly to the SEQV4L PCB, you could save a little bit of vertical space here, i´d say ca 7mm…
Also, 14mm @J15A, this is with the cables pressed very flat, so it would be better to add a few mm