There is a 1/2" x 4' x 8' white building board on the market that would be easy to cut into a 4'x 7' piece and glue up. You would have to use qtr round or some other decorative moulding to help hold in place while the glue sets and to finish the edges around the beams. I used to buy truckloads of this product to distribute to the retail lumber yards in the late 90's. Armstong Ceilings was making the board and so was a Canadian outfit but I can't remember their name.
Another way would be to stip the ceiling with 1 x 3's 12" o.c. and install 12" x 12" ceiling tile. Armstrong also makes a planking that looks like 1 x 4 x4' white wood strips that can be stapled up just like ceiling tile. Armstrong ceiling tiles and planking are available at most Home Centers. I used the planking between beams in a 20 x 20 addition and my wife loves the look.
Hope this helps.