3080 & 2080ti, used a Pcie Riser for 2080ti and so far 3080 runs at 54° under full load and the 2080ti stays @ 59° (My ac is always on and electricity is included in rent ;)
I used a few of these recently for single rail pcie lines: https://www.amazon.com/dp/B09CH2LSNT?ref=ppx_pop_mob_ap_share
and my psus have 2nd CPU ports, I use these for risers: https://www.amazon.com/dp/B09LCS9T2Z?ref=ppx_pop_mob_ap_share
also, I've used other types of splitters as well. COMeap has some good shit
I also buy alot of compatible cables instead for my psus, which will be like the normal 8pin from psu to dual 8pin, but with a longer length so it's easy to power riser and gpu with same line without splitter: https://www.amazon.com/dp/B07BJ2BP3X?ref=ppx_pop_mob_ap_share