Yup, thats the way I'd do it, and I'd do it in an item so that its pen could be designed at +0 so that any/all parma resists, non-ritual, and nobody needs to recast it. Probably a few approaches:
(a) Conjure a complex image of a dense forest to surround the area in a donut shape. This is not really what the OP asked for, but it will allow a "barrier" to stop viewing the covenant. Probably the "cheapest" solution, but the target area needs to be complex. Being a created image it also affects magi, but this is better way of not doing passive attacks on Magi.
(b) Or a way would be way-markers which cast a MuMe effect at targets they detect (giving the Parma resists), and then effect them with the changed perception. Something like R: Voice, D:Conc, T: Ind with a heap of uses and the typical constant effects and environmental triggers. Create a few of these which constantly echo a very natural sound in the detection effect. Raises some questions about how many targets it can effect per round if many people come within range.
yes, those devices are casting spells directly on those who approach...such is life. With Pen =0, and pseudo-defensive, I think it is not overtly a worry.
(c) Possibly a way is further away from standard stuff. Change what the person is seeing. I did some tinkering below...
R: Touch (+1), D: Conc (+1), T: Annulus (+5, a special shape, see below)
Base effect fro MuMe is hard to suss out. Could be Base 2, to make a major change in a persons memory of an event. Or even as far up to base 4, to Completely change a person's emotions, but I thats too high imho. So assume Base 2 equivalent.
Annulus - a two dimensional ring shaped geometric area (the Latin word for "little ring"). The key reason the shape is needed is that the effect should apply to those crossing the target area, but not those within the covenant. Like a moat around the covenant, but Boundary in size.
I suppose a Torus might be a better effect area to gather those who approach who are not directly on the ground, or perhaps a hollow domed shape.
Total effect is ~25, then add all the enchanted device mods and you're probably around a 35-40 level enchanted device.
In creating that Target I think it might be described by the OP as a Boundary (+4), and then increased by +1 to incorporate the complexity of the shape; so final +5 modifier. The Annulus will have to be marked is some manner as well to be effective. In this way the effect is cast upon the Covenant as Touch by the device, but the effect is delimited to the specified area around the covenant, not inside.
Just some thoughts.