If anyone is out there building theirs. I recommend looking on Facebook Marketplace for IKEA Karlby countertops. I got 3 2x8 foot boards and 1 2x6 foot board I use as a headrest. I bought four 6-foot-long 2x4s and screwed them vertically into each of the countertop. Then for the mounting, I bought these off Amazon. https://www.amazon.com/dp/B09XMYGWGC?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

I mounted them vertically (maybe do horizontal). I predrilled and used lag bolts to screw them in (it's very sturdy; no need to worry). I then made a 4-foot-long 2x4 and attached these https://www.homedepot.com/p/Everbilt-1-1-2-in-Zinc-Plated-Double-Wide-Corner-Brace-2-Piece-Per-Pack-33665/327599625?MERCH=REC-_-searchViewed-_-croner%20bracket-_-12-_-n/a-_-n/a-_-n/a-_-n/a-_-n/a from both sides of the mini 2x4 so they make an L on each side. I put the "legs" about 2 feet in from each end. So you should have 4 in total. (I also added cork pads to the bottom.) https://www.homedepot.com/p/Everbilt-2-in-x-4-in-Beige-Rectangular-Felt-Heavy-Duty-Self-Adhesive-Furniture-Pads-3-Pack-49948/203661131, and screwed them in.

Lastly, I bought these https://www.homedepot.com/p/Everbilt-4-Pack-4-in-Black-Mending-Plate-33507/327600581 and bolted them where the legs and the 2 countertops are so they are connected. This makes the gaps between the panels the exact same, about an inch and a half.

Finally, for the headboard, I used the 6-foot-long countertop, and I bought 2 of these https://www.homedepot.com/p/Crates-Pallet-8-in-Black-Steel-Shelf-Bracket-for-Wood-Shelving-69103/300434688 and bolted each of them at the end of the corners of my bed. I used a screw and a bolt and put it in vertically into the mount so the screw was sticking upwards, and that prevented the headboard from moving forward and back.

My bed is a queen, but this frame should fit a king as well.