Does anyone know the best way of attaching a new seat cover to the (new) base?
I bought the base from KDI but it doesn’t have the sharp barbs to attach  the cover like the original Yamaha pan.
 

I would guess glue, staple gun or maybe both.

I'm very lucky to have a professional upholsterer a two minute walk from me and I give all of my seat covers to him to fit. He has also been known to cut and fit foam for me if the original is fubar.

Carter......As I recall when I put together the seat kit I bought from KDI for my DT1, it included a "rubber" flexible rod that I rolled the vinyl fabric around and stuffed it in behind the various tabs all around the inside rim of the pan (see pics)

Also, a hairdryer is very helpful in getting the cover to stretch tight enough to get a good fit

  

Maybe reach out to Dave at KDI and ask him how it’s supposed to attach in lieu of the tabs. I made my own tab strip for a seat pan with rusted tabs but that would be a last resort for this situation. 

I have one of Dave's pans and it had the "clamps"  Check with him and see if you got a defective pan!