Karla - you've created a monster!I think there are two potential problems.
The first is that lime putty/marble (as in the intonaco) behaves very differently from a concrete mixture. I can keep a premixed intonaco in a no airspace, sealed bucket for a long time. Concrete, on the other hand must be used that day, although it can be "retempered" one time only(remixed to make it soft again) after it first starts to set in a couple of hours. The lime putty in this concrete mix is only to help mix the sand and cement together. The mix is approximately one part dry cement, three parts clean, sharp sand, and one quarter part lime putty, and a splash or two of water. As far as cost goes - it is minimal. I previously recommended a "spinner paddle drill" but you could also use a large container and hoe or shovel - a "cement mixer" is for making a pourable mix, not like we use in fresco (it is weaker). There is a 'false' set within the 24 hours -don't let it fool you - the piece is not hard enough to move around - this will take three days, try to keep the piece damp. The only way to change this is to use a quick set cement, which is more expensive, to get a workable set in one day. There are also lightweight aggregates (perlite, vermiculite) - do not use them - they are too weak.
The second problem is the size of the piece you are making. This piece will weigh about 80 to 100 lbs when finished. If this seems high, without the mesh (at equivalent strength) at 2 inches thick, it would weigh 4 times that amount! So this piece will need a steel reinforcement at right angles to the direction of the hyrib - otherwise it may bend when you are moving it around. What I have been doing is waiting for the first coat to cure, I fasten 3/4" galvanised steel "channel" bar with holes drilled throough the mesh and concrete, using stainless steel bolts (about one inch long). A piece this size will need two, probably three bars.
The method I recommend is to put the hyrib rib side down on a flat table, with a sheet of plastic underneath. The concrete should not be "runny" but stiff, the drier the better but it has to stick together when you trowel it and each grain of sand should be covered in cement/lime. You are to fill the mesh until it is about 1/4 inch over the mesh - it should look like a big solid slab, trowel right the concrete right through the mesh. The outside edges should be no more than 1/2 inch away from the edges of the mesh. After it is has cured, and the bars attached (they are great for attaching the hanger straps - I use aircraft braided wire with cramps - like the sailboat rigging), take a skim coat of the same color concrete, and finish what will be the back of your fresco which will not be fully filled because of the mesh. This may not be necessary for large panels, but for small panels, where you are getting a premium price, people always look at the back, so you have to make it professional as possible. I scratch in my logo, etc., but will probably make a rubber mold of a nicer logo, to cast right into the concrete.
Once you have a collection of meshed slabs, various sizes, etc., the rest is easy, and fun. Making the slabs is the chore of the whole process, but a couple of days concentrated work can make a lot of slabs. Within a couple of months you will have arms and abs of steel (or lime concrete?) - sure to impress your artistic comrades. The alternative is limit yourself to commissioned murals, a very hard market to crack as other frescoists on this list can attest. There is also good possibility that you can get a local concrete company to make up these slabs for you at a fair price.
I promise you once you have gone through your first batch of slabs you will be hooked on fresco forever.
