What exactly do you mean? You don't want to buy from a sex shop and look for ideas?
I tried once sculpting a cock cage using glue gun bars. Check my album.
I don't recommend for long time as I'm not engineer, but it is an idea you can try for fun.
Even the genuine ones I don't wear for more than couple of hours, I prefer only when I am horny and I can't be seen by wifey.