I have taken this as a challenge. Disclaimer I have not got a copy of Dark Imperium (yet) so these will be theoretical.
So far for the Foul Blightspawn I have:
1) Use this Model:
2) Disregard this arm:
3) Add a nice skull here:
any will do this one is from:
https://www.games-workshop.com/en-GB/Citadel-Skulls-2017
4) Now take your flamer template from 7th
and butcher it so you get this:
5) Chop of the right vent and dryfit this part, DO NOT GLUE
6) Glue this arm from the Malignant Plaguecaster on:
like so:
7) green stuff a vat on the back using the template piece as reference for the pane. Also, greenstuff a tube to the bell, this makes the hose.
8) Drill out the clapper in the bell
9) Paint model
10) super glue flamer template pane.