I sat down last night and built the first Hydra last night. The quad guns were easy - Aegis Defense Lines Quad Guns. The base it sits on fits perfectly on the turret ring of the Chimera. Here's the test fit.
This last weekend I was using the quad guns on a "normal" Chimera. But I wanted to shove the guns back a little bit. Two friends of mine, RabidChild and Rain have both built these (for Inquisitorial Armies, IIRC) several years ago, for their command Chimera's. When I asked both of them if they still had pictures (armies are long gone), RabidChild responded with this link to theback40k. Which added some juice to the brain puree to make it.
I did it a little differently than he did though. Here are some pictures of it in progress.
This is the section of top hatch that needed to be cut - basically the back section is right about where the angled bit comes up on the back of the Chimera. I used the two nubbins as a guide for razor saw. With the back end of the Chimera covered, I then took the rest of the top plate and figured "about" where it needed to be - using those rivets as the guide for that side.
As you can see from the roughed in green stuff - that makes the angled bits slide in nicely with the back. I glued the two pieces together using sprue and the plate that comes with the kit when you build a basalik/griffon out of it. All the raised bits were trimmed off the back plate - the lasgun portals, rivets, and raised lip for the top hatch. Greenstuff was then added to the holes for filler.
I had to then fill in the hole left from where the angled top plate hits the chassis. More bits of griffon plate (for the rivits) and a support strut (bit of the "missing" top plate - aka garbage) to hold it in place.
Here's the "complete" model.
Since these pictures were taken, I've done some sanding and cleaning on the greenstuff and seam line. The top plate still isn't glued in (waiting till I'm happy with the seam. The flak cannon bit sits on the turret ring nicely, but I think I'm going to magnetize it just so it won't fall off too often.