One of your best and inexpensive gun mounts can be found here
Get the BSA Red Sight, its cheap and it works great and it fits on the P90
Best decent red dot sight for around $20, you cannot go wrong.
I used one on my P90 too when I had one.
The only downfall is that there seems to be a little glare from the red dot when its dark. But in broad daylight this red dot sight works great.