I vaguely remember being told that a snare drum with snares off is a suitable substitute for a field drum. Is that true? If not, what would be, and do you think a judge would say anything about it?
If you don't have a field drum, I would either stick with a regular snare with as deep of a shell as you have, or find an old powerlite marching snare and put concert heads on it.
There is definitely a different sound with a field drum, but it should have the snare sound still.
Will a judge say anything about it? If it's a solo instrument for measures at a time - maybe. I doubt anyone will have time for that nuance on your judging though. Also, I think every judge understands that not everyone has the extra funds lying around to purchase a field snare for one piece of music.
If you can find some money for one, I do enjoy having it as an option to use for marches especially!