St. Alphonsus Liguori recommends at least 30 minutes of mental prayer per day, and teaches (with many proofs) that it is morally necessary for salvation. He has a wonderful book on prayer that he believed was the most important of all of his writings, and declares that teaching souls how to pray, and exhorting souls to pray, was very lacking even in his day. He also teaches you how to do mental prayer in the aforementioned book entitled "Prayer: The Great Means of Salvation and of Perfection".

PDF here: https://archive.org/details/bwb_W9-CTR-079/page/n11/mode/2up
Physical copy: https://www.amazon.com/gp/product/B0DYYXD6D3/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B0DYYXD6D3&linkCode=as2&tag=httpwwwchanco-20 />
Devotion to the Holy Face is also quite fitting for Lent (it involves contemplating the sufferings of Our Lord, making reparation, etc.), there are many beautiful prayers in this old book on the Holy Face: https://archive.org/details/manualofthe-archconfraternityofthe-holy-face/page/n7/mode/2up

Have a blessed Lent.