Here is one of the strategies I share in From Flapping to Function: A Parent's Guide to Autism and Hand Skills.
Attach 3 Velcro Strips to a plastic backing. I used a see through envelope so that I could store the plastic pieces. I cut these out of a white orange juice bottle.
Cut smaller shapes to write the lower case letters and larger shapes to write the taller letters, upper case letters and the letters that dip below the writing line.
Show your child how to choose which size shape to write the letter and then place on the Velcro strips as shown in the photo. Be sure to use a dry erase marker so that children can wipe them clean- receiving great deep pressure stimulation to hand muscles and joints in the process ....
Source: Sensory Visual Perception Writing Activity by RecyclingOT on Rumble