is there a base pattern that I could buy to modify?
Yes and this is it.
Mccall's 5992. I changed the neckline, because it flared out, and discarded that collar. Everything else worked. What I wanted was a loose fitting shirt, without darts.
On some garments I added a back pleat, or widened the bottoms, or cut curved hems, or allowed for side vents. I added my own pockets and changed the neckline too. It's a pretty straightforward shape, and I found it easy to modify.
However, I traced the pattern in my size, and reduced the seam allowance to 1/4 inch, so I could easily stitch seams with my serger.
My first attempts were thwarted because I chose too large a size and it sent me back to the store in a hurry. Even mistakes are worth making, if it teaches me something. After 8 finished garments, I am ready to really get 'arty' with the next several pieces. And today I cleaned out my closet and filled a huge trash bag with clothes that don't work for me anymore, but are still good. They will be going to Goodwill, along with a huge bagful from Dave.