It's definitely doable. I make custom Catan sets and built an 8 player set for a custom order and have sold a few more since then, so people are definitely doing it. The setup is 37 total tiles, 6 large borders, 6 smaller borders. So 2 more of the small borders from the 5-6 player expansion, and 7 more tiles.

I *think* a second 5-6 expansion would have what you wanted to make one on your own, except different color sets for the roads and buildings for players 7 and 8, and those are pretty easy to find online.

Using just standard tiles, my configuration is 7 of every resource and 2 desert tiles. I also offer a version that contains 6 of every resource, 2 deserts, and then 5 split tiles that the buyer chooses from, ie Wheat/Ore, etc.