Here is the outline of the bracket I made after I bought seats for my Camry (Gen 4) as reference.
Purchased Per Seat:
(2) Angled slotted pieces
http://www.lowes.com/pd_215675-37672-11138_4294934474_?productId=3053569&Ntt=metal&Ntk= i_products&pl=1¤tURL=/pl_Hardware_4294934474__s?Ntk=i_products$Ntt=metal
(2) Flat slotted pieces
http://www.lowes.com/pd_69812-37672-11140_4294934474_?productId=3059255&Ntt=metal&Ntk= i_products&pl=1¤tURL=/pl_Hardware_4294934474__s?Ntk=i_products$rpp=15$No =30$Ntt=metal
Screws, bolts, and washers. (forgot size)(Blue/Red Dots)
(1) Tension washer (Purple Dot)(Yellow Dot Setup)
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Green Dots: Original Seat Bolts
Red Dots: Racing Seat Slider Bolts
Blue Dots: Bracket Rail Bolts
Yellow Dot: Original Side Seat Bolt

^^The metal bar with 6 Blue Dots is one long piece that is bended to create a "spring tension" effect to support weight. Think of weight distribution.
Try and push it down to see if it repels your force.
When bending, try to avoid sharp bends, it might sag from the weight.
Make the bends smooth.
To take pressure off the Yellow bolt, the bend of that side needs to rest on the floor, like the side near the door with the bolt on the floor.
Green Dots: Original Bolts
Red Dots: Racing Seat Slider Bolts
Blue Dots: Bracket Bolts
Yellow Dot: Original Side Seat Bolt. You can use this to secure the buckle bar. It wont move with the seat, but its secure.
Purple Dot: Washer that "Fits perfectly" onto the screw and has to fill up the gap for the buckle steel bar. Only way to secure the buckle without being loose/wobbly.
Needed if you are going to use the stock belt buckle.
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Hope this gives you ideas.
I did the measurements on the fly...
Some pre-slotted holes will need to be filed to fit screws/holes right.
Corners that needs to be bended inward or sand/cut to keep from scraping against. (Sharp)
Takes some thinking to make this work with your seats.
If you want, I can take some pics of my bracket for you.
And yes, if i had the right tools, i would use stronger steel.

But hope this does get you started on planning yours.