A number of the big board maker shipped their own backplate solutions for this class of boards, though Tyan doesn't seem to have been own of them 🙁
My Asus NCCH-DL & NCCH-DR boards both came with the X-PAD accessory kit, with adhesive backplates plus various heights of threaded nuts and rubber pads (these replaced the CEK springs)...
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...and my Iwill Dh800 came with a full size backplate to be screwed to the chassis...
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You also used to be able to get similar generic solutions like this - hard to find today though...
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If making your own, watch the height of the tapped threads (may have to add small threaded spacers) so as to apply sufficient load when the heatsinks are screwed fully down