Can you set the speed of the ethernet connection in the xbox. If you could force the ethernet to 10mbps in the setting it would accomplish what you want.
There is not simple way to do this with say just a cable. The cable would have to be so bad that the devices can not negotiate a 100mbit connection. You can actually get 1gbit connection on cat3 cable over short distances if you are lucky so it is not as simple buying some cable.
You could buy a 10/100 switch BUT it would need to have some ability for you to log in and set the port speeds. You do not want to spend a lot to get a fully managed switch but you might find one that has some basic abilities....need to read the manuals before you buy I suspect.