if you want to put the compass to the mast, be aware of the fact, that the Lasermast turns around in the maststep and so your lovely Silva 103 P will turn around with it together.
And there is a lot of stuff (cunnighan system and outhaul controlline, vang) at the mast between gooseneck and maststep. I think it is better to put your Silva 103 P in a box (for selfmade electronic facilities you can get from your local electronic-shop) and bolt the box on a Laser deck plate. Be aware of the old / new control line systems between centreboard and maststep, so buy a box that is a little bit higher to be able to drill the wholes correct for putting the controlline(s) through.
Last but not least: Also contact SILVA! The 103P seems to be converitible to a 103R. The bracket+base used for the 103R may just fit over your 103P (maybe with minor modification), converting it into a 103R. Then, you could use the standard Silva compass mounts for the Laser. (Let us know if this turns out to be an option.)
You can also try to just get a piece of 5"-wide, 1/4" thick "L" shaped plexiglass plate and build your own mount. Straight plexi base with an epoxy + screw mounted mahogany as the vertical part will also do.