How to make a concrete seat

Appendage: Easier options

Easier options albeit for a less exciting seat.

Discard the angle fillets

You can do-away with the angle fillets for a plainer look around the top of the seat.
If you do-away with the angle fillets you must run the bevel cut on pieces (c) and (d) through the full thickness of the wood.

Discard the angle brackets

Use clamps instead of the brackets to hold the leg forms to the seat form.
You will save sourcing or making the brackets and fitting them, and also save from having to fix the connecting bolts to the seat form.
The downside is that without the brackets and by using clamps, you will have to manually measure and position the units as you go. Okay for a one of, but probably not much chop if you intend to make a few.
The brackets on the leg forms and the connecting bolts in the seat form make for a quick and accurate alignment when bringing them together during the casting process

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Discard the fancy shaped (e) pieces

You can replace the fancy curved (e) pieces with straight pieces, say 2 ½” x 2 ½” (62mm x 62mm) stock or bigger.
This will save the time and effort of making the curved (e) pieces.
It will also save a little time during the casting process, as you will not have to dismantle the leg forms when positioning them around the reinforcing.

The downside is that the legs of the finished product will be straight, and pretty much characterless.

Discard the plastic sheet

The plastic sheet is to give a smooth finish to the top of the seat. You can do away with it and just use the plywood.
If the plywood surface is smooth, then so too will be the finish on the concrete (although probably not as smooth as with the plastic).
Concrete will take on the same finish as the form itself.