Hi, welcome to Ribnet!
The fillets are usually made of an aluminium extrusion of some description - the main aim is to keep the floor panels rigid & flat.
If you are under 3m, and can't find official spares / replacements I reckon a couple of suitably fixed timber or Alu battens would do the trick. There was a thread In the SIB section from a guy who had built a bespoke floor like that & used the battens to mount stuff on.....
I guess the big Q is - do you plan to fold it up each time? If not (i.e. park it on car roof inflated) then any permanent