I looked into this recently - if you type homemade reptile incubator into google loads of ideas come up... I went to a local pet store and he showed me one which he had followed from a google search. From what I remember, you need
polybox (the type of box that fish might be shipped in)
heat mat
thermostat (matstat)
hygrometer
2 bricks
Heatmat (attached to matstat) on lid of box or underneath the box - thats a personal choice really, water in bottom of box (not too much), 2 bricks on base of box with a grill over the top - you place the eggs in small tubs of damp vermiculite and place the tubs on top of the grill, place the hygrometer into the tub and thats about it.... I havent made one myself but I have seem a finished example - you may not even need the water in the bottom of the box depending on the humidity you are getting.
T