A backing only increases Set on a bow.
If I understand you right you basically want to make a horsebow without all the effort of Horn and sinew.
If thats the case you should use Rattan Manau. If U only want siyahs, Take like 120cm of Rattan and add the siyahs. However this doesnt really Look Like a horsebow, so U can bend a setback of 20° into the handle with a heatgun. Since Manau is a Bit slow U can/should add a facing of 2-5mm. Anything thats Common for facings will Work. Walnut, yew core, Cherry, etc. If U Like perfection U can even add a raehidebacking, it gives u aber little more Speed, a Fee pounds and most important it prevents the Manau fibers from straightening Up. Since rawhide is flexible it won't cause Set. So:
Manau
Siyahs
Setback in handle
Facing
Rawhide