Your TBH is beautiful! I have Les Crowder's book and several TBH plans, and I think I could build one (not any worse than a chicken coop...right...) so I am getting some lumber and messing about next week with this. The window...I want one of those too!
The windows were ESSENTIAL in my viewpoint. The whole point of this endeavor was to be able to see what the bees were up to! How can I see anything without windows?
I put four windows (2 each side) - much like you'd build a storm window. I cut a groove in each side of the boards and slipped a piece of plexi into it. (tongue and groove like you'd build for a panel door).
The thing I'd do differently - for the ones I'll build this spring's use - is to make the windows flush on the inside of the hive. The bees have used the little ledge (2mm) to build attachments for their combs! If the window is flush with the interior, then the follower board is also flush, so the bees don't skip around the follower board and start building in the back of the hive!
My sons enjoy looking in the windows to see what they're up to. Now that it's cold out here, it's good to be able to see the cluster and make sure they're doing ok.