BioMax
+1y
The distance from the back of the diff cover is set by your confines. You would want it as close as possible for strength reasons, but there isn't any reason for it to be super far away anyhow.
And as far as to make your own or not is completely up to you. If you want to buy one, then find one that you like. I have no idea who has a decent one because I would build one of my own.