I've been known to go 8088 to 8087 internal w/out the 'adapter' just through a slot in the back
or with no IO shield it fits through there too... I only know because I forgot to put the IO shield on and used an external RAID card to my expander, to fanout to test the cards.
I'd just have gotten a backplane with an expander in it, ie: a cheap supermicro 24bay off ebay
then its just plug and play, and go, no converters, extra cables, fanouts, etc... probably ~same $$ after you add up the cables, fan-out cables, internal to external converter, and sell the chassis you have. (Or just replace backplane with expander one)