Hi, If your hair is as thin /wispy as you say, all you need to do is mist the top /crown area with water and slick the hair flat-down against the scalp....... then cut a long length of cling-film and lay it onto the head, tying it under your chin [not too tight !] --don`t pull it too tightly or it will tend to `shrink` when you eventually untie the bit under your chin and give an inaccurate size /shape.
Just add a few more layers of material and sellotape [there are plenty of Vids on `Youtube` on template-making] .. once you have it reasonably correct size [by using a marker to draw the outline of the thin/ area needing coverage on your head] and it`s sturdy enough, just untie the bit under your chin and trim the excess material off......it`s best to wait a few days and try it on again...it may have changed a bit [shrunk] so you may need to adjust it by adding on a bit more material here and there ....this way, you get to keep the hair you are presently working with.
Templates for a topper don`t have to be accurate to the nearest millimeter...you just need to give a representation of your skull -shape and an outline of the area to be covered... hope that helps and good luck with it !