Legal in every class as padding. Structural tray changes are not.
This is one body's answer, not the sport's. You are bound
by whichever body sanctions the event you enter, and where both govern you,
you are held to the stricter of the two. The differences are set out in
the rulebook comparison.
Freestyle uniquely allows widening a Ski standing tray by up to 4 in total (2 in per side), and allows footholds installed into Ski footwells. Ski GP allows integrated foot holds and rail caps provided they do not extend beyond the bond line.
Where it stands, class by class
20 classes, grouped by the answer rather than listed one by
one, because an answer that holds for a class usually holds for every class
that borrows its rules.
Legal, within limits 20 classes
Padding and/or mat kits may be added in every class. Where footwells exist they must be BLOCKED OFF during competition with no indentation into the footwell sides.
Footholds, tray mats, padding kits: if the rule your class applies reads
differently to you, or the rulebook has changed since this was read, ask.
We answer with the section we read it from, or that we have not read it.
Ask a rules question.