You have tried. there is no point. They just don't get it. One that creating an image of God is forbidden, imaginary or carved, and that prayer is not a platform for imposing our imagination on scripture and on God, but a time for him to inform our thinking. They insist on transcendental meditation. They will never be convinced other wise.
'They insist on transcendental meditation' you say. Have you read 'Thou shalt not bear false witness against thy neighbor'? That is in the 10 commandments, too. Since I brought it up, let us look at the first of the... or first two of the... ten commandments, depending on how you count them.
Exodus 20
2 I am the Lord thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
3 Thou shalt have no other gods before me.
4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth.
5 Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;
Notice the command is not against making images 'in your mind.' It is against making an actual graven image or likeness.
Now consider this passage from Ezekiel 1:
26 And above the firmament over their heads
was the likeness of a throne, in appearance like a sapphire stone; on the likeness of the throne
was a likeness with the appearance of a man high above it. 27 Also from the appearance of His waist and upward I saw, as it were, the color of amber with the appearance of fire all around within it; and from the appearance of His waist and downward I saw, as it were, the appearance of fire with brightness all around. 28 Like the appearance of a rainbow in a cloud on a rainy day, so
was the appearance of the brightness all around it. This
was the appearance of the likeness of the glory of the Lord.
(NKJV)
If someone reads this and pictures in his mind the throne, one who appeared like a man with the appearance of fire from the waist upward, with rainbow-brightness all around, is this idolatry?
Why doesn't Exodus 20 forbid imagining a picture when praying? Why does Ezekiel 1 contain this description using visual language which leads the reader to imagine what Ezekiel saw?
I do not try to picture God when I pray. I generally do not, except when I read a passage like this. I do not encourage others to try to picture what they think God may look like. But I cannot find any Biblical basis for condemning such a thing as idolatry. IMO, creating laws like this is akin to what the Pharisees were doing with their hedges about the Torah, creating their own burdens, restrictions, and traditions. I consider this to be in that category rather than the category of associating lust with adultery in Matthew 5 because it is implicit in other scripture, for example Ezekiel 1, leads the reader to have a picture in their mind of this type. Revelation has a more detailed description of Christ, also. The Pharisees erred by forbidding what was allowed and allowing what was forbidden.