The Misumi T-Slot HFSQN4-15250-500 was very popular for the Shapeoko 2 when two pieces fit the machine as if made for it:
(esp. during Misumi’s “Reddit-150” promo)
I don’t know that anyone has done it for an SO3 or XL or XXL — the cost stacks up pretty quickly — I believe that in addition to the pieces running front–back you’d need additional structure going side–side. I tried working up a B.O.M. for extrusion and parts available from Fastenal/Grainger (trying to avoid the added expense of shipping) but have instead continued w/ my weird T-track/MDF/PVC hybrid (at least I went to the effort to spray my MDF baseboard w/ spare urethane).
https://forum.shapeoko.com/viewtopic.php?f=37&t=8332&p=65068#p65068
(though things have changed since then, integrating the C3D T-track in addition to what I was using — only had 2 track left over)