darth,
The number one thing to bear in mind is that it takes time. If you don't 'get it' even after this thread has been filled with sage advice, that doesn't mean you should give up... just keep pressing on.
Anyway, assuming you've got that, here's a few more ideas:
1. It's widely agreed that a good solo
isn't just pointless noodling over the top of the rest of the musicians. Playing scales is a starting point but you want it to sound like an expression, not an exercise.
2. Don't take too long a solo - 128 bars is a long time to fill with something interesting

On the other hand, something like 8 bars is relatively easy to give structure to.
3. Can you get a recording of the backing you'll be playing over? Take it home and just sing over the top of it until you come up with something you like and then try to figure out how to play what you were singing. There are a lot more technical barriers between the imagination and making a sound on the bass than there are between the imagination and your voice (allowing for the fact that your singing is in private and doesn't have to sound tuneful or pleasant to anyone else!)
If you want a short 8 bar solo idea, how about using a form like ABAC
A - one bar of the underlying riff and then a couple of notes that give some contrast
B - one bar of the underlying riff and then a few different notes... if you went up last time, come down this time
A - exactly the same as the previous A... this is helping your 'solo' have structure
C - some kind of variant that contrasts with all of the above.
You could also post the riff you are working on here - either a link to a sound file or some tab to give the general idea.
Wulf