I can show you how to make them with pictures
the cost to build them is (2) passenger seat brackets +
Goto home depote buy 4 of the 24" thinner of the rebars for cement steaking about 1/8th - 1/4" thick about 2-3$ each cut it at 18" exactly drill your 2 3/8 holes for your 12mm bolts sparco milano seats use 12.mm bolts.
bolt the beams centerd onto the bottome of the seat.
bolt down you passenger seats rails tight with the beams bolted to your seats get a tube of JB weld put four dabs of it on the bottom of the seat rebar beams then set it on the seat rails where you want the seat. clamp it down let it cure after its cured pull the seats off the beams and the seat rails out but with the beams lightly welded onto the seat rails with the JB weld then hit it with a real welder and go all the way around your jb weld area and weld it permanently in there it will never come apart again if done right this way.