Here's a quick Basilisk-to-Medusa conversion I used for my Pre-Heresy Luna Wolves army. It was based on this picture at Dakka. Unlike the Dakka version I had no spare Storm Sword barrel. Seems like a pricey bit to use for this, so I substituted in a small piece of PVC which I cut down to about 3/4", de-burred and sanded smooth. Here's really all you need:
The Basilisk gun, the smoke stack from the Manufactorum, the case from Valkyrie multiple rocket pod, the bit of PVC and some very thin plasticard.
First hack up the Basilisk barrel. You'll need to cut off the long part of the barrel and remove the section that is connected to the upper support. Also cut off the top of the smoke stack, smooth it out and attach it to the rocket pod and PVC tub. When completed, it should look like this:
Now just assemble the Basilisk cannon back-half with the new barrel. You'll probably have to remove some material below the upper holder so you can get a snug fit. In the end, the upper tube will be resting on the section of PVC.
Now comes the tricky part. Cut a strip plasticard long enough to fit around the barrel of the Basilisk you cut off in the step above. You're going to make a sleeve that fits inside the smoke stack and around the barrel. Dry fit it a few times until you get the diameter and reveal (how much card is sticking out of the smoke stack) just right. You'll probably have to clean up the cut end of the Basilisk barrel to get a good fit.
Once everything is satisfactory, slather the inside of the smoke stack with glue and cram the plasticard and barrel in there. Try to line up the seam with the bottom, so it is less obvious. I wrapped a small rubber band around the plasticard to help it keep a cylindrical shape. Try not to get glue on the Basilisk barrel so it can be easily removed later.
If you end up with a small open seam like I did on this one, you can add a small strip of plasticard and file it down later. The armor shield will have to be cut and trimmed to fit around the PVC, but once it is complete it should all go together just like the Basilisk.
That's it; pretty simple if you have all the pieces. The best part is that this can be a Medusa or a Basilisk by adding the long barrel. Medusas are great in one-on-one Heresy games, but you'll probably want to break out the Basilisk for Apocalypse games because of the better range.