Hi
1128/29 come in basically 4 parts the knuckle, the deflector,the spring and the keeper, The knuckle and the deflector parts have a 'T' bar so on Peco wagons you would remove the rapido coupling and then replace with the MT ones. this way you use the original coupler pocket should you need to go back you can.
1015/16 or 1125's the spring sits inside the coupler and there is no 'T' bar so yes you would need to make them up with the coupler box and remove the one on the Wagon, admittedly if you was to buy the ready built versions of the 1015/16 it would be easier but at the loss of not being reversible.
The 1128/29 or even 1130's are a little fiddly to try and hold together I admit but a little patience and good eyesight to see where the springs have gone (them springs are tiny) I find the best way is a tiny amount of superglue in the spring keeper then using tweezers to hold the two coupler parts together into the keeper ease the whole thing into the Peco pocket and then slide the keeper forward until it won't go any more (easier to do than explain) then a dab of superglue at the back of the keeper to just hold it in place and your done.
The amount of superglue is tiny so won't stop the rapido couplers being replaced if needs be, why you would want to I not sure exceptHi
to sell or P/X the wagon
Cheers
Mark