With miniature omnis, boundary mounted to a vertical wall, I simply gaff tape them in place directly to the surface, no stand or mounts required. I do have some hard rubber boundary mounts made for the minature DPA omnis which work nicely and protect them from being crushed if accidentally stepped on when used on the floor. The rubber mounts can similary be taped to a wall or even a person.
Rocks, your construction looks similar to this recommended Shopes accessory: http://www.schoeps.de/en/schoeps-recommends/turtle
, which both suspends the microphone using elastic bands to eliminate solid-born handling noise (vibration transmitted through the floor) and offers protection from being trampled upon, although it looks like the capsule portion protrudes from the protective housing and is vulnerable. That can be used for directional microphones as well as omnis, and that blue superCMIT shotgun sticking out right at foot level looks very vulnerable to me!
You want to get the capsule as close as possible to the surface for best results. I've used non-minature omnis placed directly on neoprene rubber 'mouse-pads' laying on the floor with a sandbag (actually they were lead-shot bags) on top, which damped vibrations and also protected the microphones. Obviously the capsule has to protrude enough not to be obscured by the sandbag, and the polar pattern to the rear is somewhat modified by the sandbag.
Volt, for boundary mounted recording of the marching band out on the field, you'd want a vertical boundary like a wall that faces the field. You might be able to setup two stands right up against a large wall facing the field, and arrange the mic stands so the microphone capsules are only a 1/4" or so from the wall surface, facing upwards or to either side. With foam windscreens on the mics you could push them right up against the wall. If both omnis are mounted on the same wall, then you'll want to space them as you would A-B omnis in free space. If you had a triangular wall corner that points towards the center of the field (doubtful, but who knows), or are using two pieces of perspex (plexiglass) or plywood or a couple folding tables or whatever as boundary surfaces which you can arrange however you like (anything hard, flat and large enough will work, the larger the better, 4' square or even more is best) you could arrange the microphones closer together with the two surfaces angled away from each other like directional microphones. That will introduce more level difference and less time of arrival differences between channels.
If you'd rather not hear them marching left and right in the resulting image, back and forth to opposite ends of the field, spacing them A-B more widely on one surface will get that open spaced omni bigness and ambience (appropriate for this I think) without such hard panned imaging effects.