Ok, here´s a small walkthrough on how I did my Sentinel in order to have it look similar to my 2 drop sentinels... (Yeah I know... I should have bought 3, but somehow my mind didn´t work with squadrons at the moment of buying...)
Anyway, I´m a pretty lazy person, so I wanted to have a sentinel which passes as somehow related to my Elysians, without being bothered too much with green stuff and plasticard. So I grabbes a sharp modelling knife, a rasp, a standard sentinel kit, some parts from my bits box and cut out 2 pieces from any GW plastic kit sprue.
This is a qucik and dirty conversion. Most parts fit nicely, but be prepared for lots of rasping and or sanding.
Part list:
1 Sentinel
1 Valkyrie pilot head (or any other head you wanna use)
1 Baneblade heavy flamer tube (again any tube will do as long as it fits)
1 Valkyrie side sensor panel
2 cut out plastic sprue parts (these will form the rollcage)
Lots of random bits box parts that fit your style

I started out redoing the engine parts to lower the silhouette of the sentinel. Like on the >Elysian drop sentinel I made a lateral exhaust.
step 1: cut off the exhaust as near to the base as possible. Try to leave everything as intact as possible, you´ll need most of the parts later on ( Im really lazy!

)

Step 2: Take the left (referring to the left one shown in the last pic) exhaust and cut it in three parts:

->

Step 3: Take the remaining exhaust.

Cut it in half carefully. You can cut along the mold line. (did I say to do it very carefully?) And cut off half of the exhausts tip.

Step 4: remove the flamer from the tube.

Step 5: Assembly of the exhaust area:
Take the 2 outside parts from the first exhaust and glue them together. (You can put the middle part in your bits box...)

->

Be carefull to have the exhaust facing upwards (in direction of the 2 rivets hopefully still be visible on one part)
Now glue half of the second exhaust on top of the engine housing with the boxy part facing to the right. After that take the 1st exhaust (which should be very small by now) and glue it on the right side in alignment to the exhaust on top.
At last, take that tube and glue it on the remains of the original exhausts position. (I didi this mainly for looks. You can use whatever you like to cover up^^ )

Step 6: The main body. Now it gets rough.
Cut off the poles from the main body. Be carefull you need at least 2 of them later on.
The Lysian Sentinel is much lower so our pilot has to sit lower himself. To achieve this you have to cut the pilots body in half and ( in fact you can use every Cadian, or Elysian, or Catachan or whatever body) add the arms. (You will have to sand them in order to have the pilot fit.) Use your taste in which position you like the pilot. I moved him a bit nearer to his door like in the Elysian ones. This involves lots of glue and try and error. (I did say I´m lazy? I´m sure there are ways to make this perfect, but this is the quick and dirty method;-)
Assemble the Sentinel drivers cab and add the pilot.

Step 7: The rollcage.
Cut out two appropriate parts from a plastic sprue. They have to be long enough to form the outer parts of the rollcage. Again use try and error... GW supplys us with lots of plastic on the sprues so you can try a lot. Now bend them 2 times and sand the ends so that they are level. (now my writing skills aren´t enough... please refer to the picture...)

Only to claryfy: You need the same parts two times.

!All directions in this step are from the front perspective.!
Now glue these two onto the sentinel and add two of the poles as square stabylizers. They should now sit left and right with the two poles on ahead and one behind the pilots head. That leaves you with a gap in front of the pilot and left from the pilot. Glus somethinh that you seems fitting for you in front of the pilot as a heads up display. I used a plain plastic plate I cut to fit. The gap on the left can be filled with a valkyrie sensor ( the one that fits into the box on the rgiht side of the cockpit). This again requires some cutting on the torso and some rasping.
Finally add the engine and lots of bits that make you comfortable.




-> example of a finished one:

Thats it. Now you can add whatever weapon you want and stick it to its legs.
Cheers