I'll be starting a new instrument soon...
An Eb euphonium or Eb mini-tuba made from the body and bell of a Yamaha YEP-201 and the valve section from a small Buescher Eb helicon (bore measures around .595"). Not sure if it will use a tuba mouthpiece or a large tenor/bass trombone mouthpiece.
The body and bell need work, the body especially. I don't have a lot of tools for some of the dents, so I'm either going to be buying the tools needed or having the parts worked on by someone else. The rest of it is fairly straightforward - assemble the body so that it's in-tune with the pre-existing valve section in Eb.
In conjunction with the above, I'm starting another bass horn using an Eb Buescher helicon - it's the perfect size. The bell is 17-inches in diameter and the body is quite small, slightly larger than the Barcone body I used previously. Best of all, it has a very nice taper. I will either use a couple of different universal leadpipes to make it almost entirely conical or have a custom leadpipe made for it. The mouthpiece will be a bass trombone size.
Haven't quite figured out what key I'll put it in, but CC looks promising. I think I have another rotary valve that I can add that will have two slides -one to drop it into BBb and the other to act as the more standard long whole step.
Hand-stopping will be a little more difficult, given the larger bell, but I may some how try to figure out a bolstered leather glove of sorts to use.
I have something close to a lead on another Buescher Eb, I just need to bug the owner a little more!
I'll try to get a picture posted of the Buescher body and start another post on the Eb euphonium