Two options spring to mind looking at the Hotrod Octa;
1. Buy another complete speedring set - but may have to shorten the rods a bit(?) I have Stripboxes, Softboxes & Octoboxes all different makes/sizes and I readily interchange the speedrings with no issues.
2. Buy a speedring adapter, 4x nuts, bolts & 8 washers - drill some holes in the adapter and just bolt it to the existing speedring - cheap, but you won't get any rotation.
Based anywhere near Plymouth, got a few spare ring/adapters and should have an 'S' fit you could try.