If it is more than 12 inches deep or so you’ll need plywood or pressed wood. I think 12 inches deep is adequate. You can still have it 18 inches long if you wish.
I got an expensive piece of either pine wood or oak and made sure there were no knots in it. Go to your local Home Depot, look at the different pieces of wood available, go home and design it. Home Depot will make straight cuts in the wood but no angle cuts so you’ll need your own saw.
Before I put in the screws I drilled a pilot hole (about .25 inches wide and .25 inches deep) and after putting in the screw I filled the cavity with wood paste (or whatever it’s called).
You can then sand down the entire surface so it is smooth before staining the wood and glossing it. I recommend using 2 or three layers of gloss.
Interestingly, I was 14 or 15 at the time and couldn’t wait to get to the Bais Medrash in the morning so I could use the shtender.
Good luck.