One of your best and inexpensive gun mounts can be found here
http://www.dealextreme.com/products.dx/category.914
Get the BSA Red Sight, its cheap and it works great and it fits on the P90
http://www.dealextreme.com/details.dx/sku.12211
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.