As one who prays grace before meals wherever I am - sign of the cross and all …
No it is not sinful …
But why are you reluctant?
Anyway … I read a study once by people who study things like linquistics and cultural superstitions … you know … Why is it bad luck to spill the salt and where “crossing your fingers for luck” come from … and other such things … I wish I could link to it … but this was nearly 40 years ago … long before Al Gore invented the internet
Going back through the various cultures on where and why “Crossing your fingers for luck” entered the scene … what they believed was that when the Christianity was still an illegal religion in Roman Empire and Christians were under persecution - they tried to hide certain “Christian” practices when out in public as it were … thus rather than make the sign of the cross on forehead, lips and heart - they would cross their fingers … as a form of self blessing - this sign - covertly done became associated with a desire for a good outcome - or “for [good] luck”
This if those early Christians could covertly cross their fingers - long enough to enter into the cultural norms we recognize to this day - I doubt the practice was sinful …
If I were you though - I would just cross myself and be who you are - a Christian. specifically a Catholic Christian
Other items in the article discussed the spilling of Salt - they found no references to that in history before Da Vinci’s painting of the Last Supper was famous … in that painting Judas spills the salt … as we know he leave to betray Jesus and then commit suicide - they concluded that the painting was the source of “Spilling salt” being bad luck …