Ok, here is my King size foundation/base build with pictures. The material list is above. I used framing hardware such as corner brackets and 2x4 supports which is probably overkill for this, but I wanted to ensure it was build sturdy. The brackets are inexpensive and are more reliable than just using screws to attach all the frame pieces. One thing though, you will probably have to make the holes in the brackets bigger to accommodate screws. these are usually held in place by nails when framing up wall, decks, etc. I just picked a drill bit big that was about the same size as the diameter of the screws. You don't have to do all of the holes, as you will see below I only used 3 screws on each side of the corner bracket. That is more than enough to keep everything in place.
I started out by measuring from foot board to headboard and subtracted about an inch to have some wiggle room when I put the base in place and cut the side pieces to this length. Next I used a router to cut about an inch wide groove in the top of the side rails just deep enough so the slats would lay flush across the top. (this took some trail and error to get the right depth)
Routed out section for slats
If you don't have a router, you could just lay them on top of the 2x10 but I'm a little OCD and this would've bothered me even though I would never see it..... LOL
The I put both side rails in place and held them with a clamp and got my side to side measurement, once again subtracting a little bit for some wiggle room and cut the head and foot pieces to length. Once I had all 4 of the 2x10's cut I put them together using corner brackets. These brackets aren't usually a perfect 90 degrees so you have to bend them a little to keep the frame square, I suggest using a speed square or L square.
Corner Bracket.
Finished frame
Once all 4 sides were together I cut the slats to length. I used a 3" piece of wood as a spacer to speed up the process of getting them set in at 3" spacing.
3" spacer
I opted for using my air nailer over screws to keep the slats in place because it's a lot quicker, and you don't really need a strong connection for them anyway. It's just to keep them from sliding back and forth.
All the slats in place.
Once all the slats were nailed in place I flipped the base over to attach the 2x4's that ran head to toe. I used 2x4 framing brackets to attach them for added strength. Measure the distance
inside the head/foot 2x10 and cut to length (The Brackets extend down the length of the 2x4 so they don't have to have a tight fit.) I spaced the 2x4's like so; one in the middle, then put one on each side of the middle one splitting the difference. Also check for a "crown" or bow in the 2x4, If it has one make the bow side up so when weight is applied to the frame when it's all done it won't sag. When you attach the brackets make sure you put some pressure down on the 2x4 to ensure its right up against the slats so there isn't any sag. Also remember, the frame is upside down so you have to install the brackets upside down too (see picture below) I didn't worry about putting any screws in the sides of the bracket to attach them to the 2x4. It will be wedged between the bracket and the slats so it isn't going anywhere.
2x4 brackets
When that was done, I flipped it up on it's side and put nails in the slats where they went accross the 2x4's. Not necessary, but once again my OCD felt the need to do it. Finish nails are cheap anyway....
In place sitting in my bed frame
I did add 4 small pieces of cut 2x4 in in the middle of the head/foot board and on both sides because of the weight and the long span. Is it necessary, or just more overkill on my part? I'm not sure but I didn't want to take any chances of the side boards breaking. You can barely see one of the them in the pic above, in between the 4th & 5th slat on the left side. You can see them, once the mattress is on but I'm going to stain it to match the bed frame to help blend in. I would rather be safe than sorry...
I would say from first cut until last nail it took me an hour & half to 2 hours. Biggest pain was getting it square in the very beginning, but it's also the most important. Hope this helps, and if you're at all handy and have the right tools, I would suggest building versus buying your own. Most places I saw wanted $200-$500 for King foundations and I guarantee this is much more strong and reliable and only cost about $100.