Maybe this wouldn't work, but I've been thinking of Borg ships that aren't either spheres or cubes, and don't require massive scale to be intimidating, and I was just rewatching the Enterprise episode "Regeneration" the other day...
The "Borgified" Arctic research vessel that Enterprise encountered had a really sinister look to it, and it's the only completely assimilated alien ship I've ever seen in Trek. It could be an excellent alien vessel for the NX-01 to be displayed with, assuming DST could produce it economically.