IMHO take it to an RV shop or RV dealership, CW, etc. These guys have done tons of them and are very experienced at it.
For me, when the ordered base plate listed for my car didn't fit THEY took care of getting the correct one and making any adjustments because they had the experience and had seen it happen before.
Many forum members have had luck doing at a local auto shop but it's just 'my' advice to go to an experienced RV shop. Just saying, it definitely was a plus for me that my car was at an RV shop.
We must be willing to get rid of the life we've planned,
so as to have the life that is waiting for us.