Very happy with the results of my attempt at PPs Megabus IDE adapter overall. Both interfaces I've built work absolutely fine, and now I have a patched TOS 1.04 which allows booting from IDE as you can see here:
https://www.youtube.com/watch?v=x1ZtnMSxu8c
As you can see, the FDD is empty.
The version of TOS 1.04 had already been patched to show TOS version, RAM and number of floppies, though I'm not entirely sure how I came by it in the first place. Nevertheless it was useful because Christian states that the IDE boot delay in his patch is quite short and maybe not long enough for drives needing time to spin up, and the other patch introduces a delay in a similar fashion to the memory test in TOS 2.06.
Since I was also intending to use spare laptop hard drives, having the extra delay to allow spin up was useful, plus pressing the space bar does allow for it to be skipped if you want.
Christians patcher is excellent, because it also allows you to split the ROMs into a 2 or 6 set at the same time as patching, which is incredibly useful if you need to burn a new set once the patching is done.
Here is the installed adapter:

- MegabusIDE.jpg (145.42 KiB) Viewed 4852 times
And the installed ROMs:

- TOS104_IDE.jpg (129.5 KiB) Viewed 4852 times
If anybody is interested in building their own, and patching the ROM, here are the links I used:
Megabus IDE:
http://www.8bitchip.info/atari/megastide.html
TOS 1.40 IDE Autoboot Patch:
https://github.com/czietz/ideboot/
Patching and splitting the ROM is easily achieved using Hatari, I guess there is another post in that
The only part you may find awkward is the Megabus connector. What you need is this:
Harting - 09021642825 DIN 41612 Signal Connector Female 64 Pin Z2088
The rest of the parts are easily sourced. I ordered the PCBs from PCBWay, and they were very quick, arriving in 7 days, and the boards were high quality. Be aware they have a minimum order of 5 boards, though as a new sign up, I got a voucher that covered the cost of the PCBs, so I only had to pay for shipping. All told, 5 boards + shipping came to £22, which to me works out ok. It was good to get the experience of ordering the PCBs too.
You will need to figure out a power connection for the board and the hard drive, however the Mega does have a peripheral power pick off point, so it shouldn't be too difficult. The only other connection is ACSI pin 10, but that can be picked off the internal ACSI header.
Once the world returns to sanity I will probably build the 3 remaining boards and put them up for sale.