This is one of those little areas where I wish the plans were clearer. Two schemes are shown. In the actual plans pages themselves, it a bolt through a short section of bushing stock welded into the seat frames at the intersection between the seat back and the seat bottom serves to anchor the lap belt. This arrangement places the bolt in "single shear", which is actually a combination of shear, bending, and tensile loading and is structurally inefficient.
The addendum sheets show a different lap belt mounting. .090 4130 sheet is bent around the fuselage cross members. In order to use the belt set I had (from Aircraft Spruce, with aluminum end fittings, for Rutan Eze's), I'd also have to induce a 90 degree twist in this piece to get everything to line up. I couldn't do it with the tools I have on hand, bending hot or cold.
When I went to Oshkosh several years ago, I took many pictures of as many details as I could think of. Only one shot showed lap belt attachment, and I didn't like the way it had been done.
I wanted a lap belt attachment for my IIL that didn't interfere with the pilot's feet, loaded all members in a structurally sound manner, and could be fabricated using the tools at hand. The photos (under construction) show what I came up with. It's basically a .060 4130 steel channel welded into the side of the seat back/bottom intersection. There's a little tab on the outside of the channel that gets bent inward and welded to keep the channel from closing up under load. I think .050 would be thick enough. A short AN bolt pins the lap belt end in place. The flat pattern is also shown (not yet it isn't), although you should develop your own based on your seat.
I had many pictures of the shoulder harness attachments, the most popular method wrapped the pilot's belt around an additional cross member forming an "A" at the forward turtledeck bulkhead. Again, due to the nature of my belt set, this approach wouldn't work. I found some pre-bent stainless steel attachment fittings at the marine hardware store that I can wrap around the cross tubes as per the addendum design. They're nice and shiny, light and expensive. For the second set I'll need (passengerŐs seat), I'll try again to bend 4130 into a circle like the addendum drawing, this time using .060, and if that doesn't work, blow another $20+ on the stainless pieces. They're used to attach roller furling line lead blocks to lifeline stanchions. Schaeffer is one common brand.