Anything is possible... the first Sproggy I put together had a PCB that I made myself with it's own design. Same schematic, just wanted to integrate a few other things.
The current PCB designs that are around here are small... I don't know if there is any room for improvement in the total area of the board. The ones Mastero sells are long and skiny.... So I am sure if you want to spend the time you could make it shorter and wider.
I used AutoCAD to draw the traces and pads, printed on vellum and used that to expose a photo-etch board. It is not hard but takes some time to layout all the traces and make sure everything is right. And cost wise you can buy one cheaper from Mastero than you can by a blank photo-etch board.... And it is already drilled, which is a royal pain in the *** to do
I still don't know how he does it.