OP, you are high on crack. Please go hire someone to do this work already.
Several have suggested reinforcement. Metal grid, rebar or mesh in the concrete mix. Since you aren't building a fucking bridge you don't need pre-stressed cables embedded.
You can buy straw at the local hardware store. They usually have a bale or two kicking around the lawn and garden supply area. You need this to insulate the concrete while it sets up. Of course, you could use something else. But straw was suggested because it is fairly easy to remove and biodegradable. You could also use fiberglass or dirt.
And yes, you can do this in a garage for the initial cure. After 24 hours, you can install the cover. Then you cover it with the straw so it is below the frost line. At 2' below grade, it _never_ freezes here but you can't leave it exposed to the night sky our the concrete will freeze. But it can cure slowly over the next couple months below grade. Try to focus when sequential steps are provided. You can't just dump everything into the hole and hope it forms a cover.
No lid is going to prevent ground water from getting in if this is a 2' below grade. You could place a seal on it but that isn't going to help much.
You want a 3' disc of concrete thicker than 1.5" (which you say is too thin). Well, a 3' disc of 1.5" thickness is already 132 lbs. So, no you can't make a lid from concrete that you will be able to lift out of the hole without lifting equipment.
http://inspectapedia.com/septic/Septic_Cesspool_Design_Strengths.php
The above provides the loading factors you need to make your cover.
As for the forms, the suggests of existing lids are a good idea if you didn't need something that is likely 4" thick. You are going to need to build this out of wood. That's traditional and time tested. You could also build it out of anything you would like. But since you likely have a hammer, wood is the choice. Yes, it is going to cost money to build the form. You can't get around this. You don't need finish grade material though. 2x4's nailed onto a sheet of exterior grade plywood should do the job. Nail into a rough circle. Cut with jigsaw into a circle (You might need to buy string and a pencil to mark it out -- Please allocate a few dollars for useless string and pencils).
Or, TL;DR -- I need to make an $500 cover for $0.05? How do I do this?