We supply many company's deliberately non Europe with longer side seals so the builder can grind the ends to get the best fit for each groove.
With the grooves cleaned and lightly oiled you should just feel the side seal move to and fro under your fingers without measuring, this is how the Mazda engine builders put them together in Japan by feel
Only available out from Japan or America via Japan, most term these longer seals as "Mazdaspeed side seals"
For the ends, just replicate the side seal ends you take out.
Here is a link that may help your re-build https://www.racingbeateurope.com/b-rx8-rotary-internal-engine-partsb-459-c.asp
Hope this helps