For a simple but effective way of protecting tender plants from slugs or the weather; take a 1.5Ltr or 2Ltr clear plastic bottle and cut off the base with a craft knife. Remove the stopper and place over a single plant.
You can also remove the top of the bottle and slit lengthways to produce a mini plastic tunnel. Use flat pieces of plastic for end caps. Some companies sell plastic mouldings to maintain the shape.
If you want a bigger ‘mini greenhouse’ then try using 5 ltr clear mineral water bottle. Cut off the base, remove the cap and use over plants as above.