I have been using 4 inch pvc pipe under my back while benching, but when I bench without it my back straighten out flat. I have a good arch while benching raw, It's just that I can't get a good arch with the shirt
If you can bench with a 4 inch piece of PVC pipe under your back, god bless you. I benched once with a 2 1/2 inch piece of PVC and my back was fucked up for days! It is very hard for anyone who squats and deadlifts heavy to really get a great arch. You are on the right track with the PVC pipe! Good luck!
Just make sure that once you get your shirt to where you want it and you are going to put a belt on to keep the shirt in place, make sure before you tighten the belt that you throw your shoulders back like you are arching then tighten the belt. That will help because if you tighten the belt and don't arch before you tighten the belt, there is no way you will be able to get in your arch! Make sense? Try it!