[lunarhighway]

i don't think the pattern will make a measurable difference vs smooth, most factory undertrays are terribly uneven, mostely because of structural reasons.

a full undertray will be much smoother than nothing so it will help to a certain degree even it it's not perfect. weight is an issue and when it comes to undertrays i'd say the lighter the better, bending the edges upwards (and if your material is thin enough you can do this by hand over a straight edge) will create a much stronger shape than a flat sheet, with very little added weight/lost surface, and a sheet half as thin can be half the cost.