I have a Pi 3B+ and got a gen2 FLIRC case for it a while back as I loved the passive cooling (at least the idea of it).

I've been hemming and hawing about whether to mountn it to the frame or not (whether the weight up top makes a difference) but finally decided to do it. I saw this mount, but I wasn't in love with it:

• https://www.prusaprinters.org/prints/2014-flirc-case-mount-for-mk3
• https://www.thingiverse.com/thing:3572286

I also saw this cover, which I liked a bit:

• https://www.thingiverse.com/thing:2571763

So I designed a new "mount" that was a bit of a mix between the two. To be honest, ultimately nothing from the former mount was re-used (except the measurement between the mounting holes), but including it here as it was inspiration.

Few quick notes:
This replaces the screwed in cover on the FLIRC case. The cover is extended out a bit for support. It has bars across the bottom for extra support. The bars are cut out to still provide full access to the USB ports, and I think the ethernet can be accessed (though I haven't tried).
I removed the cross bar that goes under the SD card on purpose, because I found that pulling the card out in order to put the case on / take off was a bit annoying.