Seems to me that part of what's fun with TBH is that there are few hard & fast rules. You still have beespace, but you don't worry about it much. The top bars need to be about 1.25" wide in the brood nest and perhaps a 1.5" for the rest of the hive, but everything else is open to what you want to do as long as there is plenty of room for the bees. I use 20" top bars, but not because it means anything. I just cut the first ones that long, so I've stuck with it so they will work in all my hives. I have straight sides because I wanted to try some Jackson top bars and I'm willing to cut side attachments where necessary. Other folks love their sloped sides. Truthfully, I don't think the bees care one way or the other, but that's only my opinion.
I have two big hives that are 40" long so about 35 top bars fit in it. I have two others that are only 30" long and I use them for lure hives. But the top bars fit in any of them. Incidentally, I carefully measured my big hives to accept 35 top bars, but as soon as that damp south wind blew in, one of the bars woudn't fit because the wood swelled up. I'm down to 33 laying flat, with the 34th laying sideways to fill up the final gap.
Some folks make telescoping tops, but lots of us just put something flat over the top of the hives. I hold a piece of plywood down with a brick on each corner, plus I have a strap I buckle over it in case of thunder storms.
I have hardware cloth over the bottoms of my hives, instead of anything solid. The bees don't seem to care. I made my hives so I can slide a piece of fiber-board in the bottom and I use it to put candy and grease patties on, but I keep it away from the broodnest. The screened bottom is supposed to help with Varroa mites, but other beekeepers have solid bottoms.
So enjoy yourself! Get creative. Make something wonderful and fanciful (like Wojtek's photos of the Polish TBHs), or make it totally flat and unimaginative like mine.
"Before I speak, I have something I'd like to say. . . . I will try to keep this short as long as I can." Yogi Berra