I just used resin and glass fomr the local auto store.
Apply resin to board, lay on glass, work more resin into the glass?board. Let it set.
If you reinforce the side nearest the roof (outside), the inside will still be smooth.
If you do both sides or the inside, for any smoothing I just used a flap wheel on an angle grinder. 'Light' pressure is the watchword!
Get fabric from Jo-Ann's fabric shop. Use 'proper' 3m adhesive for the job. Anything less will let go as soon as the inside of the car gets hot.
Anything I might say is probably worth what you paid for it, so treat it accordingly!