Thanks folks. They're all made from junk so if you can find stuff the right size and shape you're all set.
Quad barrel turret: the base is a little cable tie point, that you would stick to a wall to keep a lamp cable or something from hanging loose.
The main body is just a bit of carved pvc foam from when I had my gutters replaced. The shape is like a box with the top front edge cut away and the lower rear one too.
The barrels are just thick wire. I found a large paper clip that was thick enough and cut some straight sections the same length. The ends are ball point pen ink tube bits. I jammed the wires in and cut the end off the tube and drilled a couple of holes in each side. They're mounted on some plastic bits from my bits box. I angled the mounts to give it a bit of character. Top barrels narrower than the lower ones.
Then the two armour plates on the sides were just straight out of my bits box.
The green one is mostly made from a UK child safety plug. You know the plastic covers you put on an electrical socket to stop kids poking in the holes. The smaller pins make the guns and the larger pin was cut down into the same shape as the quad barrel body. A couple of cylinders to attach the weapons on each side and the whole thing mounted on a spare marauder foot (hence the choice of mech in the photo)
a couple of wires on the back are glued in for the power feeds and some fine detailing on the weapon casing is thick paper.
The white dome turret starts with a bottle top, a Robinsons fruit shoot lid, the top of it is cut off to get the reinforced disc. Then I found a plastic dome the right size. It happened to be the lid of an Olbas oil inhaler. It's about the size of a chap stick lid. Anyway I cut the end off and cut a gap in the side for the guns. I also used the lip on the opening of the tube to cover the join.
Then I cut down some space marine scout shotguns flipped them upside down and glued them together then they were glued inside the gap and some detailing was added to the outside of the dome with paper.
There's a little four leaf clover on each one. I found these lying about and thought they'd help my new guns get some lucky shots.