Poster board would reflect the light well, but its not easily cleaned. Unless you have a bunch on hand already and no other use for it, it seems a pricy way to go for what may only be one seasons use.

I know that I may be a grubbier grower than you, but between spills and general dust, my grow shelves need a good cleaning every season.

You are right. This is my 1st year doing this & I did price out poster board & it doesn't look economical. Any other suggestions?

You could look at FRP (fiber reinforced plastic) panels. I used them to line the solid walls of my greenhouse. They come in white and can be scrubbed clean. They're more expensive than mylar, but are less flimsy and would last practically forever. I found them at a local big box hardware store.

Look up Melamine at a big box store website. Sheets of it are 4'x8' and a little under 1/4" thick. It is used on bathroom and kitchen walls. If you go looking at a store, it will be with the paneling.

There is also foam core board available at art supply stores.

Another idea is to use white fabric from a fabric store. Easy to hang and if it gets dirty you can throw it in the washer.

I was looking into this a few weeks ago and got this:

Foylon: reflects 95% of the light

Mylar: reflects 92-97% of the light

Flat white paint: reflects 75-85% of the light

Emergency Blankets: around 70%

Aluminum Foil: no more than 55%

Panda Film: The white side is 75-90% reflective

C3 anti-detection film: 92-97% of the light

I made a video about using mylar wrapping paper as a reflector. I'll link it below.

Here is a link that might be useful: Reflective material video