I'm continualy amazed at how many things I figure out when I am trying to sleep...
right now it is a surface. to make it solid it needs to be "water tight", meaning a closed volume.
you can close the top and bodttom with more tspline surfaces, which might be nice if your going to do more mods to those areas. I can screen cast that easier than explaining it.
OTOH, if it's good the way it is, and you just want a solid, exit the sulpt environment. you'll note your tspline is now a brep surface. since the openings are simple and fairly flat, the "patch tool shoud work to close it up.
change to the "patch" environment (top left of the tool bar, you rprobably in "model" right now. the word patch here is a misnomer. "Surface" environment would be a more meaningfull name)
select the "patch " tool
select one of the open loops, and it should put a surface patch (don't worry if the normal of the surface is facing the wrong way right now)
do the same for the other loop
now you will have 3 surface bodies in your object browser. these need to be stiched together.
select the "stich" tool, and select all three surfaces. where the surfaces are touching will be a green line. hit ok
you will notice that the 3 surface bodies in your browser window are now a single solid body.
