24.5cm x 34.75cm that is the closest I can get on the touratech rack. I am guessing they have a very non-standard size to have this be 'proprietary'; it is difficult with the rounded edges, so this is by 'eye'.
I give you kudos on trying to build your own and yes, these are expensive for a few bent pieces of metal, but you are paying for the engineering. I have the Zega Evo X for the 2022 SAR, so this measurement is only on the side that is flat. I choose to purchase these as I like how they cut back in on the silencer side, giving a symmetrical look.
I did have to modify the black plastic rack cover pieces as it looked a little funny with the bolts exposed and wish they at least came with a template of where to cut as I over cut a few times. I may order a new set and tape where I now have a gap, so I can get these covers to fit more tight. There is still the problem of taking some of the 'girth' off behind where the frame mounts that has them stick out a little. I know it is a tangent, but wish they just made a replacement cover to it has a good fit and finish to a set of panniers that are $2k USD.
Good luck with your build as the angles for the bars to connect to between the passenger foot pegs and their footpeg mount seem to be a very weird angle. Again, paying for the engineering.
Are you going to buy the branded pannier and just want a home-built frame? Because I believe the panniers from touratech are $1000 ($500 ea).