What would happen if the tappets were set to tight?
The valves will open earlier and close later, allowing more air and or fuel into the cylinder. If the clearance is too small, the valves will not close entirely and might burn.
There are some glues in caulking tubes that would probably work. Not Liquid Nails. You would have to place and glue the bottom, then the sides and cut lengths of wood, 1x2 to wedge between the two uprights. Tight enough to hold them tight against the wall or shower enclosure. Letting the glue set at least overnight.