In Enigmundia, the equivalent of the Roman Empire -- the Imperium -- had a policy of integrating the portfolio of the gods from their conquered lands into their pantheon. To that end, all my suggested patron pagan gods per spell will be Roman.
Detect Magic (1) & Read Magic (1)
Analogous to the Greek Pallas Athena, Minerva is a virgin goddess whose aspects include the portfolios of poetry, medicine, wisdom, commerce, weaving, crafts and magic. She can therefore be the entity tapped for any magic spell, but given that magic is one of her portfolios, she is most often the one called upon by ancient rote rituals for these spells.
A signifier tends to be present when spells are called upon for this effect. I suggest the ghostly image of an owl hovering near the wizards when the spell is cast.
Hold Portal (1) & Knock (2)
Janus is set over doorways, doors, gates, beginnings, endings, and transitions -- he can be tapped for the ability to hold a given portal.
For each spell, the spell caster calls upon a specific aspect of Janus.
- Hold Portal calls upon Janus Clusius (Janus as the Guardian of Closed Doors)
- Knock calls upon Janus Patulcius (Janus as the Guardian of Open Doors)
For signifiers of Janus, normally the door is sufficient -- but at higher levels, a ghostly spinning coin is sometimes visible during the casting, showing the two faces of Janus on either side. It stops on the aspect of Clusius or Patulcius depending on the spell cast.
NOTE: I updated this post because I woke up this morning and noticed that it kind of trailed off. Never post under the influence of pain medication, I say.