I am happy to say that after twenty years of experimenting I have found the optimal cassock experience.
The most practical way to wear the cassock is with the simple (one ply) plastic collar that has the black bib in the front (rabat). You can wear it over a collarless white shirt (I buy the best bargain cotton dress shirts from any store and have the overhanging collar removed, leaving the lower part of the collar and button intact) or over a T-shirt, etc., however you wish. You can wear it with the cassock or with the gilet (clerical vest and suit). This style of collar is better than the double ply style which attaches to the shirt in the back for several reasons.
It is easier to clean and lasts much longer.
No hassle attaching it to the back of the shirt (it simply snaps on itself to close around the shirt, neck).
You can wear it with any style undershirt therefore.
It stays well enough in place if the shirt collar, the rabat collar, and the cassock are all properly fitted to your neck size.
N.B. The cassock is the best clerical garb because of it's tradition, elegance, accessories (e.g. biretta, cape, ferriola, etc.) and practicality (the fastest way to go for an emergency sick call: goes quickly and comfortably over pajamas!)
They should have told us this in seminary. Or, better, we should have worn this in seminary (as they used to do!), and they would have saved me much grief in finding the best fit!