Make the legs as normal, make the cockpit as normal.
The rear secttion is a plas-card box; dimentions-
Top plate- width 42mm, length 35mm,
Side plate x2- width 36mm, top length 35mm, bottom length 42mm, rear step 10mm,
Bottom plate- 42mm squared.
Rear angle plate- the bit from the chimera used cut in half with a bit of plas-card to cover the gap.
Forward plate- 42mm by 36mm.

Cut or file the top plate so that the coms section of the sentinel cockpit sits slighty raised. Cut the forward plate so that the cockpit sits flush.
Turrt ring is a 25mm slot base from fantsy, this was used to give it hight, add a little bit more plas-card for your desiered hight (I just added 1 1mm plate here).
Take the sponsons (If you are using the new kit ones than luky you as you can swap and change tham, if using the old kit the I'd recomend having the heavy bolter oppion,) from an old LRBT and glue them flush with the forward plate (mke sure the rivet sections stand proud a little so you can the cut some thin plas-card to carry it on in to the main body).
If you can use a large square base from fantsy (the 40mm base, If not find some thing aprox the same size) and glue that to the bottom plate, add another 25mm square base (make sure you center it on the leading edge of the big base,).
Cut a dozer blade conetion runner to either side of the 25mm base to not only hide it but to add a tech like look (ypur looking for the bit that looks like a pistion and rod section).
Cut a 10mm x 20mm plate and glue to the big base to the underside of the cockpit. Use a filler to plug the gap. to conect the hull to the legs I used a 3 inch wood screw (heat drill...or just drill the right size hole threw both the hips as well as the hull, try to get it as far forward as you can but don't go mad,), use a small washer or spacer here if you go alittle wrong here.
Leg plating are 14mm x 14mm (x4), with 5mm x 14mm sides (you'll need to file these to fit on the outer edge but if you've goten this far and uderstood my directions then this an't a problem )
The main turret...you can probably use a chimera modded to fit two auto-cannons (from the weavy weapons team models), or Las-cannons if you want a cooler look. That said, mine is a custom build and I for the life of me can't remember how i cobbled that together
Hull weapon is of your chousing but you can use the sentinels own Las-cannon or heavy flamer out of the box, or take a spair heavy bolter from some where and cut it down to size. I used a heavy stubber from a chimera kit and glued that under the cockpit on the right of the Las-cannon just so it keeps the look balanced between the gun load outs.
Add stuff from your bits box to make cool.
The base is from the Revill snap together X-Wing model. Its larger than the standed base at 87mm, if you cn find some thing that size (most hobby shops will carry woden cutouts close to that size.
Some gluing, some cutting and Bob be your uncal you got your self a Mk4 Battel Angel