Don't panic, but you have a bottom mount seat and side mounts.
For a quick fix I would use 4 (spare) angles and bolt them tightly to the bottom of the seat with the horizontal bolt hold facing in. Hopefully you got some extra parts when you ordered the chassis. Then loosen the rails on your frame so they are just inside where these angles come down and bolt directly to that.
Or you may want to bolt your seat straight to the slider if the holes match up and then bolt the slider down to the rails. You didn't post an image of your slider so I don't know how that would work.
However it looks like you may have a plate that fits your seat and would also allow you to bolt straight down to the 8020 profile. So you likely have a few options.
If I remember correctly, I drilled holes through the side of the rails that touch the 80/20 and then mounted the L brackets to them. I then used T Nuts to mount it to the 80/20. I hope these photos help.