Shipbucket

Another very interesting concept (although slightly awkward-looking with those thin "legs" of boosters and empty space in the middle).
I'm not really well versed in the arcana of space technology, but would it work if instead of these long boosters You used more of them, but shorter and more densely packed - like 5 of them in X shape (4 on corners, 1 in the middle), or 6 (placed like bullets in the revolver)?

I don't see why not, and I agree with your aesthetic criticism. The Shuttle solid rocket motors had thrust vectoring for attitude control during launch, which is why they had those fat skirts around the base (compare and contrast). In a two-motor application, one can imagine that either motor might not have full-azimuth authority. Rather (for example) the right-side motor might not have a nozzle that could vector towards the left side of the vehicle, and so you could mount the motors closer together.

However, (1) this design was derived specifically from the 8.4m system and so I did not have the freedom to relocate the hold-down devices on the base of the solid motors, and (2) aerodynamically it would be suboptimal but totally okay.