I use a scrap rack from any car. Welded plate on the bottom to either clamp to bench or put 56lb weight on, then add threaded bar to spline end so it takes any type of depth wheel using spacers to accommodate variances.
Add also a grub screw into top with Allen head and key to stop it free spinning like a brake.

I once had a customer bring a classic wheel to me that he had removed all the cracked Bakelite plastic from hoping to get it redone. when realising the cost he asked if I could help and my solution was to get rubber hose with same diameter hole running through it to what the wheel rim substrate was and cut it down the length rebonding it onto the rim with areldite making the join at 6 o’clock for leather seam. Once set I used body filler to level out spokes and then recovered it in hide. Looked spot on and worked great. Hope that helps with your ideas

Don’t know if pic uploaded sorry.
Firstly I’d just like to say I don’t like to pick fault with anyone’s work as we all have off days.
However I don’t know if this will help you but there is a fault between the two sides of the Squab (seat back) as the diamonds are not matched (iv circled the area in image) so wondered if maybe you had taken up too much Seam on one side causing it to sit wrong.
My method would be as previously mentioned and dry fit against cushion and mark your centre point so when cover pulled on you can reference it.