Side entry cage on the seat tube is probably the easiest and cleanest. If you need some security in gravel mode you can always put an elastic on it.
I’d drill and rivnut inserts where I needed the cages but I know that isn’t what most would do.
DIY version of what AndersDaag posted would be to find some 1/2- 3/4" wide 1/16" thick aluminum from a home store. For the downtube I would bridge the upper hole on the lower bottle mount to the lower hole on the upper bottle mount. At the lower of those holes you mount the lower hole of the downtube bottle cages, and then mark the piece of aluminum for the proper bottle spacing and drill a hole. Use a button head type screw from the back side with a nut on the cage side. For the downtube you do similar mounting the aluminum to the two bottle cage holes, you can either use the lower screw for the upper bottle cage or place then where you need then. Might need a washer or two between the aluminum and the frame to make room for the button head screws from the back side. I’ve done a version of this to move a bottle forward/back between my aero bars on an old setup.
Also had one of these that I used with moderate success. Although mine was for a normal saddle and not the brooks version.
your seatbag goes through the middle of it and a bottle on each side. Unfortunately I bought direct and the company did a bait and switch and it cost me over $90.00 with shipping and customs.
I used it on one 1650km trip. It worked ok??? But I have never used it since.
I have an eBay special of them, wouldn’t use it for normal day-to-day stuff as my legs brush against bottles when loaded, but when backpacking it def works to reduce the wobble in my tail bag as well as carrying more water (or maybe beer).
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