Hi Diceys,
Basically, you could look for a straight on picture of a PRS and scale it to lifesize by taking certain known dimensions like body width, nut width etc. as a basis.
That would give you the outline of the body.
The internals of a semi hollow you could figure out by yourself: you need enough material left where the neck is set in, where the bridge sits and where the strap buttons screws will be, everything else you can leave or remove to taste.
Also take into account that you need enough material left on the bottom, to hold the screws for the covers and also to have the covers sunk into the body.
Neck body joint is more or less as on a Les Paul Junior Doublecut.
I think, you can do it like that, no plans needed in my opinion. Top carve, well, that is another story. Personally I just hit the maple with finger planes and scrapers and let the carve form itself, others prefer routing templates. Not sure how well you are set up regarding tools?
As for "legal" issues etc. : as far as I remember, PRS himself started with a copy of a Gibson. I mean, you have to start somewhere, you usually do not come up with an all original design in the first. And as this is intended to be for personal use as a hobby build, not intended to be marketed, I really don't know why that should be a problem. I mean, I also build guitar and use certain aspects of PRS and other companies, which I could not use, if I would intend to sell them, at least not in the US. But strictly for my personal use and joy, I can do whatever I want, as long as I do not make money with it.
Guitar luthiers and hobby builders all learn from one another and I think it's a good thing to have more people involved in building as that can mean: progress for the thing we all love - the guitar.
Have fun building! It's amazing to hold and play, what you build! And it is amazing to compare it to an original PRS and find all those details that they just do to perfection, that drive you crazy during the build and let you ask yourself multiple times: how is it possible for them to make these details so perfect? For me at least, a PRS is a benchmark in quality for me to meet as a hobby builder, and am always grateful for the work they have done to improve certain aspects of the guitar.
Best regards
Jonas