Too bad they did not talk to a carpenter before they designed their form. No REINFORCEMENT. I did not see them casting, but how is this concrete waterproofed? To get many re-uses of plywood forms you need to keep somethings in mind. The faster you strip the longer your form will last. Plywood is PERMEABLE and is destroyed by the ALKALINITY of concrete. Even common oil based paint and primer will double the life of plywood, if you mix that paint with 2 part epoxy form life will be over 120 casts. The smoother the form, the less effort will be needed to strip, extending the form life by preventing stripping damage. If you make the inner form fully assembled and fixed you can use car body filler to make exposed edges smooth and make sure that concrete sand from leaks does not fill the ledges in the support structure of the form. I noted them using linseed oil? cooking oil? as a release agent. Not nearly as good as 30% candles melted into diesel, applied with a rag. Trying to strip plywood trapped on 3 sides is nearly impossible without damaging plywood SEVERELY. They might be able to strip 2 inner strips of the inside form, but getting that last 2 strips out would be hell. The problem can be mitigated my making the inside a slight TRAPEZOID, placing a 2X2 chamfered 3/4 on one side in every inner corner. The plywood abutting those 2X2s should not be cut at 90 degree but 85 degrees so that just the leading edge of the plywood touches the VERTICAL 2X2s. for single sheet at a time removal. You could just make the tube PERMANENTLY fixed if you built it like a 4 sided pyramid (with the top flat) but it would have to be stripped soon, I would suggest well under 2 hours after pour. keep the outside form on until the inside one is out, supporting the green concrete. If you made the bottom of the cast (top of the tank) a square ring perhaps 3" deep you could drill the inner plywood at the top edge of the planned our, allowing a pry bar to be inserted and wedged over the outer form to lift the inner form free. To reinforce the concrete, use 12 Gage wire and run it through holes drilled in the base. Bend the wire double and stick the doubled part through the hole and bend it over, then tie all wires off at the top of the pour over the bottom of the tank. I suggest 2 holes or 4 wires per side. monitor the placement of the wire as you place concrete making sure they are in the middle of the matrix. For horizontal "bars", drop loops into the form as it is filled, positioning them with a stick, form/shape them in advance by wrapping them around the top of the assembled inner form before complete assembly. The outside form uses bolts to blocks screwed to plywood. Bolts and concrete do not go together well as sand clogs threads, over tightening will soften screw connections and there are many parts that might be lost (washers and nuts). I would instead PERMANENTLY fix the horizontals and pre drill them so that a single 1/2" pin of smooth rebar could be run down each corner. If you placed chamfer inside as stops it would also prevent leakage when the form expands. If you made the pin longer than the form a little you could make a "T" so that the pin could be wedged up and out. This also makes the horizontal braces (2X3?) perpendicular to the grain of the plywood allowing 1/2 not 3/4 ply. Your spout support does not need a full top, as you can simply mix the concrete thicker and it will not overflow, or wait till it sets a little before completing the cast. This way there is no problem making sure that the shelf is filled completely. make a partial lid, fill and remove nearly IMMEDIATELY and trowel. Lots of concrete water proofing agents, I used "Sahara" a cheap powder manufactured in Iran.