Having fun playing with fire again I see, although im 2 years late on comments.
In the 2nd & 3rd pic of the fire grate with the ring. Are you using that one to burn lump similar to how I did it in your first link (rimfire)? Reason I ask is because I like to use the exp. on the kettle side to provide a bit more air which I think allows the lump to burn more completely without much if any small pieces left over after the burn passes by. Its not necessary just the impression I gave myself after multiple test fires, before and after the fattys.
The baskets...I hate the schitty little tin baskets Weber passes off as a constellation prize for buying their grill, the older wire ones work better but alas two fires in one hole intrinsically creates instability, arguably. You have the right idea making them with exp. and making them deeper but I think they'll work better with a different shape. In my test fires with the baskets I'v made I got the best results (duration, complete burn, and potential) with a bean shaped pair. In my mind two baskets are an obsolete lo-&-slo method after the rimfire or snake method.
If the purpose for the baskets are to focus heat for high heat grilling or for cooking small a single small round basket, without a partition, will work better than the hokus pokus aluminized tin baskets. As much R&D as weber does you'd think they would have figured that out....
Also Im not saying two baskets WILL NOT work what I am saying is theres a better/no hassle way.
Maybe they work fine with briquettes, I wouldn't know, I dont use briquettes. Mine were disassembled and used on another experiment years ago.
The screen on the rapidfire is served well by adding the exp. metal. That was a damn fine idea! Im sure you've picked up on the tip to invert it? Itll take less fuel to light it and hold more fuel if you need it too. Iv thought about using some SS window type screen, I scored from BBQMAN for my dehydrator, to cover the bottom so it dont cough so many sparks into the air when you dump it. A California spark arrestor so to speak.