Placing the solar filter before sunlight hits the instrument is the correct way of doing it. You could put the filter after the instrument, provided you don't mind being blind - the concentrated energy from the Sun heats up the filter, which sooner or later melts (if it's plastic) or cracks or explodes (if it's glass), your eye(s) receive a full dose of that energy, say hi to the white cane and the friendly guide dog.
ALWAYS, ALWAYS, ALWAYS use solar filters installed in front of the instrument (telescope, binoculars, etc.). In front of it, ahead of it, on top of it. DO NOT use eyepiece filters when looking at the Sun. Do not make any mistakes here.
HOWEVER, even mounted ahead of the scope, any improvisation with regard to solar filters is bound to come back and bite you in the ass sooner or later. Look at professionally-made solar filters for telescopes, they are specifically fitted to the scope's external diameter, and have various mechanisms to secure them in place, such as triple screws, etc.
The last thing you want is the filter to fall off because you tilt the instrument the wrong way, or because the wind is blowing a little. If focused sunlight hits your eye, you will lose vision! There is zero doubt about it.
Even with professionally-made filters, you need to make sure they are secure in place after you install them. Tug them a little, see if they have a tendency to peel off and fall down. If they do, that's a full stop to whatever you're doing at the moment.
A little math:
Let's say the absolute minimum pupil size for your eyes is 2 mm. That's about 10 mm^2 area.
Let's say you're using 50 mm aperture binoculars. That's nearly 2000 mm^2 area. That's 200x bigger than your pupil.
All the energy collected by the 50 mm aperture, which has an area 200x greater than your pupil, is injected in your eye. It's like looking into the Sun, except 200x worse.
See why you must be absolutely certain that the filter never falls off, cracks, melts, or has any other sort of malfunction?