Saturn, being a defeated god, is not an easy deity for which to find suitable invocations. In The Greek Magical Papyri, the only mention of him (i.e. Kronos) was not an invocation but an evocation, in which the magician assumes the form of Zeus his conqueror.
Fortunately however, I have other sources apart from the Greek Magical Papyri. One of the most useful of these is The Orphic Hymns. The following Orphic text, despite the doggerel quality imbued by the translator, is more suitable to Saturn’s status as a deity to be worshipped per se.
Etherial father, mighty Titan, hear,
Great fire of Gods and men, whom all revere:
Endu’d with various council, pure and strong,
To whom perfection and decrease belong.
Consum’d by thee all forms that hourly die,
By thee restor’d, their former place supply;
The world immense in everlasting chains,
Strong and ineffable thy pow’r contains
Father of vast eternity, divine,
O mighty Saturn, various speech is thine:
Blossom of earth and of the starry skies,
Husband of Rhea, and Prometheus wise.
Obstetric Nature, venerable root,
From which the various forms of being shoot;
No parts peculiar can thy pow’r enclose,
Diffus’d thro’ all, from which the world arose,
O, best of beings, of a subtle mind,
Propitious hear to holy pray’rs inclin’d;
The sacred rites benevolent attend,
And grant a blameless life, a blessed end.