You're not going to find anything pre-existing; it'll have to be heavily customized from an existing horn.
I'd start with a curved soprano on a neck strap. this way either hand will have all four of the fingers fully avaiable and the thumb will be able to do a certain amount. There's no way to hold a straight soprano with the LH only and it's not easy with RH only.
Then basically additional linkages and key touches have to be added to enable the various notes. Some notes (bell notes and maybe palm) will probably have to be forgone.
If you have the left hand working, for example, you can use these existing fingerings: High F and high E with front F key as normal, Eb, D with usual palm keys, C# C and B as normal, Bb with bis (roll on and off), A G# G as normal. Now convert the low C# key to F#, low B key to F, low Bb key to E; then palm keys without octave key give you Eb and D, then same as above, down to low E. So with just adding a few linkages you get high F to low E. For the RH it'd be different but similar.
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