Hey Matt,
I used the same lathe for the small enclosure.. what I did, because of the limited amount of space, and because the walls were made of 1/2" plywook,d I molded the lathe itself, without any framework, and fastened it securely, so that there was no lag, or flex in the lathe, directly to the bare plywood walls.. If the lathe is affixed tight enough, you shouldn't have a problem with it.. I have also used 1/4" hardware cloth, but because how the holes are shaped on lathe- deeper, and at a slight angle, it seems to hold cement and other aggregates much better.
As for aquariums, yes, probably not the type of method you would use for an aquarium, however, I have made GFRC panels, which were molded out of latex molds of authentic rock outcrops, which can be made to size, and fit perfectly in any acquarium.
*one thing that I think I should point out, is that if you are planning on using this in an enclosure that will be exposed directly to water, ie. submerged, waterfall, etc, and you have animals in there, that may be affected by a decreased pH, I would suggest cycling the tank, before putting any animals in, as cycling for a few weeks will eliminate the naturally low pH that the lime will naturally create.. There are also some liquid membrane applications that are non toxic to coat the cement with, if this could be a big problem..
Cheers...
Bob
treemonitors.com