hello there, I've not much expertise but this item

https://www.amazon.com/Extender-Bitcoin-Litecoin-Ubit-Adapter-Ethereum/dp/B073W9KCFC/ref=sr_1_19?ie=UTF8&qid=1521851957&sr=8-19&keywords=pci+riser

works very fine under simplemining resorces... can use at least two of them and you'll go 8 gpus... good luck