I understand there are items known as cast aluminum tooling plates which are what I would consider to be reasonably priced:

https://www.midweststeelsupply.com/store/castaluminumplateatp5





Aluminum Cast Tool & Jig (ATP 5) plate is vertical cast, stress-relieved, machined plate providing sound dimensional stability with minimal or no distortion even after extensive machining operations. Cast Aluminum Plates have a precision machined surface for superior flatness and thickness tolerance and are protected with PVC plastic on both sides. Cast Aluminum Plate typically has a surface finish of 25 RMS or better. ATP 5 has a tensile strength of 41KSI a Hardness of 70HB and a Yield Strength of 18 KSI.

I did a quick search on the meaning of 25 RMS and didn't get useful information to my alleged mind. Other 3d printer forums have referenced this particular item as being a good choice for a print bed due to the flatness inherent in the product.

I tossed in figures of 8" x 10" x 3/8" thick and got a price back of US$14.28. Would such a product meet your requirements?

Also, would you be making available your work to other Pegasus owners? I have a 3d printer, a mini-mill and other tools and may be capable of constructing one for my Pegasus.