Hi,
I use a 49mm tamper and it works fine. After you put the coffee in the basket you just put the tamper on top of the coffee and you apply very little pressure. Then you tap the portafilter on the worktop once, that will make the coffee grinds that have not been tamped fall over. Then you tamp and twist. Done.
I do think a slightly rounded tamper works better though. Less problems with channeling. The original plastic La Pavoni tamper is convex for that very reason. It is also slightly bigger than 49mm.
It does lack the weight though. So, If you're going to have one made, I'd go for a 49,5mm convex tamper. Otherwise use the tapping technique as described above.
Good luck!