Hi FWGW,
If I haven't overlooked it and to my surprise, your rack doesn't contain any filter? So I would use the last 8 HP for a multimode filter. 8 HP isn't very much, by heart, Doepfer comes to mind but I am sure there are other interesting multi-mode filters that fit within 8 HP too. Another option might be the TipTop Audio - Forbidden Planet, nice little filter!
Otherwise stick with Farkas' advice to put a 8 HP blind panel first, till you damn sure where the last 8 HP will be spend for ;-)
Yet another option is to stick with the blind panel option but save the money you would have otherwise spend for an 8 HP module and keep saving money till you have enough money for a second case. Once you got that second case you can stop worrying about your last module because you have enough space left! :-)
Good luck with choosing a nice filter (or a fancy blind panel or saving money for another rack) and kind regards, Garfield.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads