: How would you go about making a glass copy of a sit on top cockpit?
Making molds its fairly easy, you can use specialized materials, like for example alginates, or use what the hardware stores offer (silicones, foams etc)
The materials you will use for mold determine the procedure, release techniques included.
To go very simple and very cheap you can use expanding insulation foam, the one in the can, it doesnt stick to the kind of plastic shopping and garbage bags are made of, those can be a little tricky to mold so that they follow the curvature of the cockpit but if you have some practice with that kind of things its not difficult. Using release agents like various kind of wax, silicon spray (ive used even the non stick cooking spray for some things and worked pretty well) can be a way too. Checking on a small spot would be best since different brands of foam can act slightly differently. In short you prepare the cockpit, make a "lid" for it with cardboard or better a transparent plastic sheet held with tape and inject the foam (depending on the size you will have to make more than one hole in the lid to inject the foam from but remind that you need one hole free for air to exit the cavity you created until the end).
The foam "plug" at this point wont last forever, its good for a couple of copies or so.
A way a little more professional is to prepare the surface with the release agent, spray or brush on some latex for molds, found in any arts and craft store, if the release agent gets solid, i.e. wax, you can brush the latex on otherwise spray it.
Latex will need a number of coats to be thick enough. Once cured you can peel it off so you are sure it releases correctly and replace it in the cockpit.
Again using some foam (you can embed in the foam some wooden strips to help with this) will keep the latex rigid so its not deformed while you handle it. This one is more durable and several copies can be made from it.

The method that i consider best for simplicity result and money though is the paper marche', takes a little time for the whole to dry and be handled but thats the only negative in it. Just wax very well the cockpit, take some newspapers and cut them in strips, soak in water some strips for the first layer and cover the whole cockpit with them being careful that they follow all the contours, when soaked they conform very well.
Since newspaper ink tends to transfer, thats one of the reasons the wax coat on the cockpit is there, you can start your first layer with white paper, or even kleenexes (if you use kleenexes or similar stuff its better to place them dry and soak them on the spot with a spray bottle, be careful to not create creases)
In the meantime you will have a good bunch of strips of newspaper soaking in a mix of water and glue, carpenter glue or white glue work great and if you feel like you got the hand for it adding some polifilla or plaster powder to the water give wonderful results.
Start your second layer with those strips and layer after layer you will have a plug ready to be extracted once dry, i suggest at least 15 layers with wood strips embedded to keep the whole rigid.
Once the paper plug is separated from the cockpit and eventual creases or imperfection are filled, a few coats of varnish (or epoxy even better) will give a very durable plug. Again you can spray some foam inside it to keep it rigid and for a level, stable block.

In any mold you will find very useful to embed some tubes for compressed air to help release from the original and after from the copies (the compressed air gets in between the mold and the copy or original forcing its way around and pulling them apart)

There are tons of other ways to make a plug, some complicated, others expensive and others more or less distructive but the two above have been used many times with excellent results.

: is there a forum that specializes in making molds and glass parts?

For molds etc you can check theatrical forums, static modelmaking forums and look for moldmaking techniques for other instructions and professional forums on the net. Its not excluded that you will find some videos too, that is best since all that stuff its more difficult to explain than actually do.

If you need more clarification just ask.