I would personally join in the spirit of the prayer or grace. If you're uncomfortable saying words, then don't say them

It's better than doing the motions while hypocritically lacking a spirit of love or gratitude. If you are comfortable saying the words, then I personally see it as another part of our unending practice. We practice love by saying silly things to our children or spouses, or by making small talk to put someone at ease. So any specific concept or word is just a tool for this, unless you think it might bring suffering to others somehow.
Specifically for grace, if you're called on to do it and people know it will be non-denominational, keep it simple

Since ideally giving gratitude is good practice before any meal, there's lots of time to find something that works for you. When I'm by myself, I use a Kagyu prayer that resembles taking refuge but would be fairly opaque to non-practitioners:
The unsurpassed teacher is the precious Buddha; the unsurpassed protector is the precious Dharma; the unsurpassed refuge is the precious Sangha. To these three jewels, I make this offering.
Off the top of my head, a totally secular, non-denominational grace may resemble:
We are grateful for this meal. It is the gift of much hard work, countless people, and the earth and sky. May we stay in the present moment with each other and this food, so we don't miss this precious opportunity to eat together. May everyone be happy and meet with success in all things, and through this meal, may we in some way learn to be gentler, kinder people.
It's really up to you. I haven't been put in that situation yet, so good luck! Intention is more important than form, I think. HHDL has attended Catholic services and participated fully, as I recall.

Please take the above post with a grain of salt.