On the positive side these are inexpensive, but on the negative side you get what you pay for.  I ordered 3.  They are simply two plastic plates with sliding plastic plate between.  Unfortunately on two of the three I think the Chinese were running out of glue the day the made two of the 3. The two plates which were suppose to be glued together were not and the two plates immediately separated.  Looks likes they used a drop of glue the size of a dime.  I was able to fix by gluing together with PVC cement.  So in the end they work fine, with some assembly required.