Drink coaster holder

Being new to 3d printing. Im trying to teach myself designing so have been finding things around the house to make. i think this was a decent first go!
i only used 30% infill as i wasnt sure how strong 2mm walls would be.
As im new id love it if someone had any tips for me. and it would be awesome if you did use the design that you send me a pic :)

If i remember correct this is based on 100mm X 100mm coasters :)