It's a real faff - personally I'd probably buy a new scanner with an inbuilt one.
you need two mirrors and a sheet of glass - oh and a glass cutter.
(your first job MUST be to edge each piece of glass with tape or you'll amputate fingers)
for 35mm work on about a three inch vertical height.
the idea is simple ... create a pyramid - mirrors top ... plain at the bottom
at the apex the two mirrors connect with tape so you can vary the length of the sides
never go for an equilateral - if you do the light is focused back directly to the sensor - which is no good.
the aim is for a Scalene Triangle
I can't remember if the short or long sige side is near the scanner's hinge .... but it matters as do the exact angles.
and never
if the long/short sides are the wrong way round with relation to the direction of the light source travel ... you'll get dark spots.
I did build one years ago ... started by fixing with thin tape ... when I eventually got the angles ... I cut the glass and used gaffer tape to make it all stronger.