Record the backing progression then put it on a loop. Listen to it over and over and hum/sing/whistle along what comes into your mind. Then, work it out on the guitar. I never work with scales or think about key signatures etc. You'll find that you can hear the perfect solo in your mind but can't play it with your hands. Thats when you sit down and break it down bit by bit. A lot of people get into difficulty writing solos because they just play a load of practice runs or scales, don't fall into that category. Give it a go, it works.