The cheiroballistra text does not give precise instructions on how to attach the little ladder to the case. For some reason many scholars have been confused about the purpose of the the projecting block under the case, and about the exact form of the T-clamps.
Here the projecting block is used to prevent the little ladder from moving towards the shooter when the machine is cocked, and, with help from the T-clamps, to prevent the little ladder from sliding sideways during use. The T-clamps are primarily used to prevent the little ladder from falling off the case. The end result is very rigid and does not require inventing any parts in addition to those described in the original manuscript. The projecting block is gripped with one hand when aiming, and the elbow of that arm is supported by the pelvis. This allows a very steady aim with minimal fatigue despite the cheiroballistra's fairly hefty weight.
If we, like some scholars, had decided to not use the projecting block as rear support for the little ladder, the T-clamps would have to be way stronger than those used here. This would quickly get into trouble with the dimensions (1d wide, 3d long) given in the text. Also, we'd have to invent some other, less obvious purpose for the projecting block, as well as to figure out a comfortable shooting position.
Fitting little ladder against the projecting block Edit
First cut a slot for the rivet in the middle of the little ladder.
Chisel the slot until the end of the rivet fits tightly:
Fitting T-clamps to the case Edit
Next push the T-clamps to the little ladder bars and mark their positions on the case:
Chisel the vertical part of the T-clamp slot first:
To make chiseling away the narrow, horizontal parts easier drill a series of holes along their length:
Chisel away what's left after drilling:
Lower the T-clamp into the slot. If necessary, chisel the slot deeper until the surface of the T-clamp is on the same level as the surface of the case:
Test the T-clamp with the little ladder and adjust the slot as necessary:
Repeat the process for each T-clamp.
Interestingly, as can be seen in the above picture, the T-clamps can only be sunk into the case as deep as the centerpiece of the little ladder allows. This gives us a useful estimate for the thickness of the T-clamps, assuming they did not protrude from the case.