Redoing my basement with insualtion, 2x and s/r. To add my trim from the window to the front edge of the s/r, it is about 12 1/2". I didn't realize it was that deep and bought some primed 12" mdf, which measures 11 1/4.
So since I'm at an impasse, advise me:
Should I be using primed mdf? Would you just add a jamb extension for the extra inch and live with it?
Would the proper thing be to buy pine boards the full width? or use like interior 3/4" ply with banding?
Just want to keep cost/labor down and look decent.
I'm going to finish the trim by adding like clam shell window molding.

 

Anytime I have wide jambs like that I will use plywood. It's much more stable, and won't cup like a single wood board will. But yes, you could do it in 2 pieces by using a bottom piece that is the same width as the top and side jambs... and just add a nose onto the bottom piece. When I do I that way I will drop the nose down 1/8" and it looks fine. With plywood you have to either glue solid wood on the edge (like a 1/4" thick screen mould) or use iron on edge banding. Not a big deal.

Personally I will use wood every time over mdf.

 

I also would go with the plywood..
If you go with boards paint all sides before installing. Which by the way is not a bad idea on whatever you use.
MDF is the worst choice first it is a basement and second it is a window and MDF does not like moisture.

Get the exact with measurement and get the lumber yard to cut the plywood into strips for you.
Be sure to take a tape with you so you can check the width of the strips they cut,

 

Sure would be nice if you could post a picture of what your dealing with.
No way would I use plywood, yes it's more stable, but how would you cover up the exposed edges of the plywood at the exposure lines?
Using clam shell molding will make it look like the 1970's.

 

I would also use plywood and rip a piece of wood to glue/staple onto the edge to hide the plys.