As C-bag said there really in not enough information on what you are attempting to achieve.
first do you want the drill solid mounted so you can move the work piece into the cut off blade or is your plans to make it with the drill mounted on an arm to allow it to be operated like a chop saw.
Another big issue will be which ever method you decide the drill motor is going to be very short lived. The big problem is the design of any hand drill. none are designed to accept very much side loading at the chuck because the main shaft the chuck is mounted on is short meaning the bearings close together. In many cases the chuck is longer than the main shaft itself and by the time you mount an arbor with the cutting wheel or grinding disk on it you have created a tremendous moment arm which will quickly destroy the bearings in the drill. if any heavy cutting is done.
This is why die or pencil grinders are not used as drill motors they are designed for side loading and not thrust loading they also have much higher RPMs making them suitable for cut off applications the higher speeds require less force applied for cutting as well but They have scary high RPMs in many cases
That having been said for the occasional cut off use mounting a drill in a cradle hollowed out of hard wood then either solid mounted or using the wood as the swing arm works just fine Care has to be taken to allow enough air flow at the vent louvers of the drill or it will burn out A shaped cradle will offer good support while protecting the plastic case from breakage
You can make a blade guard out of a thin aluminum sheet just cut a disk about 3/4"to 1" larger in diameter than the cut off wheel then cut that in half you can then find something about 1/4" larger than the cut off disk and clamp 1 half of the guard disk to that then using a small hammer, fold the exposed edge over the die.do this for both pieces then rivet them together making the guard trim the center to fit over the mandrel rivet another piece of aluminum to 1 side of the guard for the mounting tab then mount that to the drill mount. BY the time you have this done you will have an idea of how to proceed further.