Ford, in their infinite wisdom, deleted the one piece seat riser after '68 on convertibles. The under structure (for torsional rigidity purposes) would be the key reinforcement to link the inner rockers together. I chopped up a once piece seat riser and installed modified regular seat risers into my '70 sports roof, which took a bunch of time and doesn't look to be worth the effort.
One thing that I would look at is using the 69-70 inner rockers vs the 65-68 versions due to the shape, which will allow more room for different seat widths. There are a couple of good reasons the one piece seat risers were so tall.
I have never "made" anything, I just modify the things around me into useful items with abilities that were GIVEN to me.