So model-wise, take an existing M113 kit, lose a road wheel, then cut down the superstructure (or scratch build some/most of it). The round rear hatch is silly and inefficient; I would just use a smaller M113 style hatch. Maybe use the turret from a V-150.
The M114 was fitted with the Cadillac-Gage turret during testing if I am not mistaken. There were several other variations on the theme that were also tested before finalizing on the arrangement that was to be produced.
As for that circular hatch at the rear, that may have been built that way to provide additional strength to the hull or it was just a dumb manufacturing decision. Always did look odd though.
I have thought about a cut down M113 on several occasions, just never got around to it. Hopefully that can change soon after my garage plus man-cave is built. :) ***
*** Garage never got built. Hospital stay and recovery from a bit of heart failure had me postponing the project. When I got back to the contractor after I recovered I was told that my $4000.00 deposit was lost/forfeit because I had waited too long on going forward with the build. So that telephone conversation ended with me launching a few expletives directed towards the contractor with regards to his intimate relations with his mother and I slammed the phone to end the very disappointing call. In lieu of a garage, I purchased a 20.0' ISO container which does work but hot in the summer and cold in the winter. But it does not leak when it rains. Yay!