Hi Pigeonpoo,
If it were me, I would not use a 'bowl', but instead, I would cut off the bottom inch and a half or so of any roughly 8 or 9 inch square cardboard box, and set that where they will be obiged to Nest, if they can not elect their own spot.
And I would gather definitely "dry" and definitely "clean" dead grass runners, slender twigs that are say 6 to 10 inches long, Pine Needles, and Broom Straws or their likes ( but no Hay as Tressa wisely reminds, ) and merely set or scatter these items for the parents to elect from for their Nest Building.
Especially in cooler climes, if one is to use a Bowl, I think it would be kindest for the Bowl to be plastic, and not ceramic or metal or other that will be a thermal mass pulling warmth from the nest. A Wooden Bowl even, would be a good choice, but regardless, I would make it a very shallow one, so the Babys are not down 'in' it in any way.
Normally Pigeons make Nests on flat surfaces...though sometimes they will elect found objects which in some way are bowl-like. But as the Babys will wish to poop over the edge of whatever they construe the Nest to be, the easier this can be for them, the better...otherwise they try in vain, and the poops roll back 'in' with them.
If you do use a deeper sort of bowl, put stuff in it so the nest that does get made, is going to be made near the top.
My most recent occasion of two recovered ferals making a nest in here, once they had started in on it in the spot they elected, I slipped a two layer, one foot by one foot piece of the small-bubble 'Bubble Wrap' under their beginning twigs, to insulate it from the bottom for them just that little bit ( since in their case, their nest was on the otherwise cold cement-linolium floor...poppa is not a flier oweing to a bum Wing...)
Have fun...!
Phil
Las Vegas