If the head of your Select SRX Freerace 42 does not fit in your board, pounding on it with a hammer... even a rubber or dead blow hammer, will only damage the fin and/or the fin box.
Get a bar of bath soap, and a single cut file.
Shouldn't take long to file off the high spots on the fin head, then the fin root will fit in the fin box
I suggest making most of the changes to the fin head because if you change the fin box, the original fin
won't fit correctly.
If you need more guidance on how to "fit an aftermarket fin to an existing fin box", just ask, although a couple of searches on the words "file" and "soap" on these forums will probably find one or more of the procedures I've written previously.
Sandpaper or abrasive cloth, or power tools are not the right way to "fit a fin".
A plain single cut file will allow you to remove only the high spots and drop the fin into the box with just the right amount of "draw" so the fin bolts really aren't what holds the fin in the box. The
engagement of all the tapered surfaces will hold the fin in the box very tightly if the fin root is fitted correctly. The fin bolts then become just a "back up", and provide the pull to "draw" the fin into the box to the correct depth.
Good idea to always carry a small (6-8") single cut file in your WS tool kit, along with a bar of soap (like you get in a motel room). The soap can be used for fitting fins, lubing mast ferrules, lubing boom extensions, anything where you want a close fit that comes apart or slides easily once the securing mecchanism is released.