Plan on approximately a 2-foot length of bench space to use for rooting cuttings. If this propagating area is across from your potting area, you can do all your work at one end of the greenhouse. To promote root growth, supply bottom warmth with a soil-heating cable. Unaffected by water, cables come with or without thermostats. Length of the heating cable you buy depends upon the size of the bed. Weave the cable back and forth across the area on a shallow bed of soil or clean sharp mason's sand and cover it with sand.
To enclose the propagating area and retain the humidity necessary for successful rooting, fasten a piece of polyethylene or other flexible plastic across the back edge of the bench. Unroll this forward to hang free over the front edge.
However, if greenhouse space is at a premium, you may want to use all of your bench area to grow plants. Then a seed flat or two filled with clean, sharp sand can serve for propagating plants. To get bottom heat, set the flats above the heater on a shelf supported by brackets.
An excellent way to handle seeds or cuttings is to convert a bench section into a propagating case. With boards, partition off an area of bench. Place a section of glass, perhaps a discarded window frame, on top, supported by the framework of wood. Or more simply, tack a piece of translucent plastic along the back of the bench and roll it forward over the area you have seeded. For cuttings, prop up the plastic with clothes pins or wire coat hangers to keep it from touching the plants.