Leeloo is compact surround speaker designed to integrate well into a living room, taking up minimal floor-space, and to provide further WAF (wife acceptance factor)& functionality in the form of a CD rack/stand. It also a good opportunity for me to explore the pros & cons of a wider baffle, shallow cabinet design. Together with Delta & Samson, a complete linear phase home-theatre system will be achieved. Being a surround speaker allows some design compromises in terms of power handling & low frequency response.
It is designed to be pushed right up against the walls to the side of the listening position - i.e there is no baffle step compensation.

Design decisions

Monopole, dipole, or bipole? Here's a good article describing the pros & cons of each. I've gone with the conventional monopole because it's generally considered the best option for multi-channel music, and it's simpler to design & construct. Also, being a normal monopole gives Leeloo the versatility to be used as a normal bookshelf speaker if so desired.

Bass-mid driver. What's required is a driver with a well-behaved response beyond 5 kHz, and a good bass performance.
Ideally, it should also have a similar sonic signature (diaphragm materials) to Delta. i.e Paper or polypropylene.
After also considering the Seas W15LY001 & Dynaudio 15w75 I finally settled on the slit-paper coned Scanspeak 15W4531G00.

Tweeter. I've decided to use the Morel Supreme 110, as used in Delta. It's easily able to handle the 1st order electrical highpass I'll be using,
and Delta will be getting a new, smaller tweeter.

Crossover. As with Delta, I'm using a 1st order crossover around 4-5kHz but here with some additional phase correction.
There will be no baffle-step compensation as it's designed to be pushed right up against the wall, possibly in a corner.
The sloped baffle assists in matching acoustic centres, and also matches the look of Delta.

Cabinet. Standard MDF materials, sloped baffle for time-alignment at 1m+ listening distance. It will also aid stability by moving the centre of gravity back toward the rear of the cabinet.Height will be such that the bass-mid driver is the listening axis, and be similarly proportioned to Delta.
The CD-rack will be able to hold 100 or so CDs/DVDs and the speaker & stand/cabinet be strong enough not to vibrate at all.
The internal volume will be about 6-7 litres depending on the crossover size & stuffing . For protection it will have a removable grill held by hidden magnets.

Bass-midrange analysis

Here are the Unibox simulations of the 15W4531G00 in a 6 litre stuffed, sealed enclosure at 25 watts.
Power handling below 80Hz will be increased to over 100W by choosing "small" surrounds in the surround processor ( which inserts a 12dB highpass at 80Hz).

Cabinet plans