I use a mix of Langstroth and top-bar hives, there are pluses and minuses to both. Most of my top bars have a front entrance at the bottom floor level of the hive and one with a top entrance in front, but I do have one hive in which the bees have established a side entrance in a gap between two of the bars and I've just let them use it that way they seem to like it.
One caveat to using a top entrance is that they tend to keep the area around their entrances clear of honey comb, so they may not build comb on the first bar if you have them enter from the top through a gap before the fist bar. They naturally don't build comb all the way to the bottom of the hive, they leave a gap, so when you build a bottom entrance they go through the entrance and crawl under the combs and the shape of the combs is un-altered. I've built hives with top entrances in the front of the hive and the bees built comb all the way up to the front, but the combs in the first couple bars had a hole through them, so I lost some potential comb area. Also hot air rises so if their entrance is at the very top, the bees may possibly have a harder time maintaining warmth during the cooler months, not sure.
Another thing thing to consider when planing an entrance, is if you are going to feed the bees or not. There are different schools of thought on feeding bees and some people don't, Michael Bush, of bush farms believes that you should never feed bees as the sugar messes with the microbiology of their stomachs. However most beekeepers feed their bees sugar water(50/50 sugar water) in the spring and sometimes fall when nectar is scarce, and some folks also feed pollen patties. The easiest way to feed bees in a top-bar hive is with an entrance feeder, so you may want to take this into consideration when designing an entrance.
With the space you have under the roof you've built you could alternatively remove some bars and replace them with a piece of wood that you cut a hole into to accommodate a mason jar and feed them from the top. However, this approach to feeding may only be useful in the beginning because as the hive grows and expands the bees will fill the hive and you will likely eventually want all of the internal space for the bees to build comb, this is why the entrance feeders are so popular.
Whether to feed bees or not is one of those endless debates in the beekeeping community, sort of like whether to defoliate or not in the cannabis growing community, and I don't know that there will ever be a consensus. I do occasionally feed my bees, but not very often like in the very beginning, or when the bees stores of nectar are depleted and only when there is nothing for them to forage in nature. Your bees will likely arrive with a can full of sugar water with holes punched in it for the bees to feed from while they are in the packages.
Good luck!!
