Hello, I also have a Marshall CODE 100 Combo and I also thought about the wheels since it is heavy to transport it,
so what I did was invent a platform with the wheels to be able to transport it easily. It is made of pine wood and
I made some perforations that were the same size as the rubber feet that the amplifier comes with and so when it is
placed on top it stays in these notches and will not fall off with movement and then I covered it with similar tolex
to that of the amp to match. The wheels have a locking system for when you need to stay fixed which is very practical.
To decorate the corners I put some metal corners and it was great.
I did all this so as not to drill the amplifier and damage it by screwing the wheels directly, but if you want
to do it directly you can, but I think it's much better to do it the way I did. 

All the necessary materials are obtained in stores such as HOME DEPOT or similar and are not expensive.

I hope you are encouraged to do so. Greetings

Here I leave the photos of how it turned out 

Gladly, that's why we're here to help.