Oups, grace's note head looks a little too big.
So here again:
\version "2.19.13"
parentGrace = #(define-music-function (parser loc myGrace ) (ly:music?)
#{
\once \override NoteHead.X-offset = #-.5
\once \omit Staff.Flag
\once \omit Staff.Stem
\once \override ParenthesesItem.font-size = #-1
\once \override NoteHead.font-size = #-4
\once \override ParenthesesItem.stencil = #(lambda (grob)
(let* ((acc (ly:grob-object (ly:grob-parent grob Y) 'accidental-grob))
(dot (ly:grob-object (ly:grob-parent grob Y) 'dot)))
(if (not (null? acc)) (ly:pointer-group-interface::add-grob grob 'elements acc))
(if (not (null? dot)) (ly:pointer-group-interface::add-grob grob 'elements dot))
(parentheses-item::print grob)))
\parenthesize $myGrace
#})
{
\clef "altovarC"
\pitchedTrill c2\startTrillSpan
-\tweak minimum-length #10
-\tweak springs-and-rods #ly:spanner::set-spacing-rods
-\tweak bound-details.left.Y #-3.25
-\tweak bound-details.right.Y #-2.75
-\tweak bound-details.left.padding #3.5
-\tweak bound-details.right.padding #.8
\glissando ~ des
\grace { \parentGrace d8 }
c8\stopTrillSpan r8
}