If you add 50 mm of foam, the Royal Enfield seat is the best choice (IMO) for a Rebel 300/500 seat. Cushion is just as soft as the Mustang, the Royal Enfield seat is wider and has more room to move around than the Mustang seat (the Mustang locks you in place).
If you are handy and not too fussy, do it yourself – if you are a perfectionist, have a shop do it.
Step 1 – buy 50 mm x 300 mm x 300 mm upholstery foam or gel pad. A smaller width like 37 mm is acceptable.
Step 2 – Remove all staples except six near the back of seat.
Step 3 – cut out foam to match the shape of seat.
Step 4 – Peel off the seat cover, leaving the last approx. 50 mm (two inches) intact.
Step 5 – (Do not remove the black plastic that is over the original foam.) Push foam into place in the seat. Make sure you push the foam into the back part of the seat. Push the 25mm strip (that you cut off in step 3) behind the foam at the back of the seat. If your cut-off piece is V-shaped, the narrow part of the “V” faces down, and the wider part of the “V” faces up.
Step 6 – Stretch the seat cover back into place. Try to work the foam for a better fit. Do not re-staple the seat cover at first. Ride the bike for a few days and see if you can push the foam into place any better. Once you are happy with how the foam fits, re-staple the seat cover.
Rebel500 