I have made two of the aegis defence line hydras and agree with Joyous_Oblivion that they look bad. I made them as a place holder until I could afford FW hydras but chances are that wont be happening either. I have been working on a scratch built hydra kit to convert a chimera that I was considering casting in resin and selling but I don't know when it will be finished (lots of other stuff on my plate not 40k related).
Buying basilisks as the basis is expensive unless you have a specific need for the earthshaker cannons. I prefer to use chimeras as the base vehicle since it's a lot cheaper and with some plastic card and any old IG players stock pile of spare track sections you can still build a chimera from what's left over.
This isn't mine but it's similar to how I made mine:
I used chimera hulls instead of hellhound so I have less armour around the back turret and it has a more open look. Mine are actually very similar to the old griffons GW used to sell which I did on purpose so when I don't need them as stand ins I can swap them over to open topped griffons later.
To build a chimera with the remains of the kit after building a hydra in this manner is quite easy. You need to trace the tracks onto thick plastic card and make 4 cut outs total. Connect them together with spare track links (no longer easy to get since the new kits only include the exact number of tracks you need to assemble that specific kit). You add detail onto the outsides of these track units as you wish but I covered mine up mostly with bags and stowage to hide the fact that they are just flat plastic. You then take the hull top, back door, and turret and assemble them as normal. The rest of the front armour you build out of plastic card taking measurements from a real chimera and add hatches and a hull weapon from your bits box. Adding rivets is much easier and more realistically scaled if you use a tooth pick to dab on super glue instead of chopped up plastic rods which are both very messy and often too large. If you do it right no one will notice with a casual glance and only when someone is really looking closely do they ask "why do those chimeras look funny?" At least that has been my experience with the 2 chimeras I have made this way.
I have done a lot of searching for other peoples work and did find this which seems close to Joyous_Oblivion's idea:
I've always liked the look of the Russian ZSU-23 (Zeus) AA tank:
I found this one which does have the similarity of the central cannons but is different proportions (but I still like it):
Cablezworth runs things like this as stand in hydras but I personally really don't like the look of them:
There is also the hydra-like turret made by maxmini. It isn't that cheap but it is less than the FW kit:http://www.maxmini.eu/store/index.php?main_page=product_info&cPath=21&products_id=118
One thing to avoid is silly oversized gun barrels like this one:
I mean, what are those, vanquisher cannons? It just looks stupid.
If you only need two or three of them it may be best to pick up some plastic card and rod/tube and make a cardboard one as a test template. Assembly line them up and it wont take an incredibly long time. You can go off of the FW one for size and since it's fairly geometric it isn't that hard. You can then raid your bits box for icons, windows, hatches, radar dishes, and other bits to add on to add visual texture.
On the left here you can see the manticore I built based off of the FW one.
I made a box from plastic card to fill in the back empty space of the chimera hull when the top armour/rear door isn't put on. I then just made a turret based on estimated dimensions and added detail as I saw fit. The radar cowling is actually the backside of the search light that comes from the imperial guard vehicle accessory sprue (no longer included with all the tank kits
). I made up a set of autocannons for this turret as well to convert it into a hydra but I don't know where they got too (probably in a box of junk somewhere in my room.) Since then I have realized that hydras are quite effective and use them all the time in regular games and in apoc games with friends. I plan on putting together at least 6 of them and an additional manticore so building each one from plastic card isn't as efficient.