I got enough interest - five people contacted me about the mini backplane - that I popped into my favorite PCB designer and freshened up the design a little.
The card fits inside a QL and supports three positions. Two positions face left and one faces right, upside down over the CPU. I don't personally recommend that you use all three positions with old cards that suck juice and spit out heat. However it does make it perfectly reasonable to have a newly minted Trump Card and QubIDE with CF slot just above it facing left. Alternatively, you could have the Trump Card face left and the IDE card face right. As long as the total current draw of the cards is under 500mA everything is fine.
The card is TINY, 96.3mm x 20.6mm. I left as much room as possible around it to allow air to circulate in the QL case. It will come with two protective plastic shields so a top card can't short against a bottom card, or an inverted card can't short against the metal keyboard backplate.
I should get the PCBs and have everything assembled in around three weeks. then I'll work out my costs and sell them at cost price. I'll post photos and so on then. I will only be making 25 of these, and have parts on hand to make 8 until I order more male DIN41612 connectors.
If anyone has requests for a custom backplane, I can do something for you.