I decided I needed a Medusa, well two actually, for a tournament thats coming up, the catch being I don't have a Medusa, or the money to splurge on a Forgeworld kit. What I do have is three Basilisk partially assembled so I decided I needed to come up with a way to field them as either a Basilisk or Medusa, given they're essentially largely the same this is fairly easy.
To do this you need:
1 Basilisk kit
1 Baneblade Demolisher Cannon Barrel
Length of 10mm Plastic Tubing (Thin)
Length of 8mm Plastic Tubing (Fat)
I'm sure you can already see where I'm going with this.
Stage one is to take your assembled Earthshaker Gun, this could even be done on a tank thats already been fully assembled allowing you to retro fit the ability to switch between these two vehicles.
Stage two is too saw through the barrel as close as possible to the breach assembly, this needs to be done very neatly and carefully otherwise you'll end up with a bad fit and gaps later on, use a hobby saw.
Now in stage three you need to fit the larger 10mm tube inside the breach assembly, push it in as far as possible, this should be about 50mm, then remove the excess, cutting neatly as with removing the barrel.
Stage four is means cutting two sections of the small 8mm tubing (or whatever else you have, this needs to slot inside the 10mm tubing neatly), this will want to be about 55mm long.
Now you can fix this 8mm tubing inside your Earthshaker barrel (ensuring that it is central and straight, check it, test fit it, because a wonky barrel will look silly! If you've done it right you won't be able to spot the difference between this finished piece and the original before you started your conversion.
You'll need to then do the same for the Demolisher Cannon barrel, once dried your good to go, Basilisk or Medusa on demand!