I have been doing this for years, for all mattress sizes. What you have to remember is that however wide and long your mattress is, for an example only say: 70 x 80 .. and the mattress is 10 inches thick. At 70 inches thick, your sheet would have to measure first the 70 inches, then twice the 10 inches for the thickness, and then you must add 2 inches to each side for the application of the elastic.
The same instructions go for the length of the sheet. Now that you have that figured out, you can draw the pattern on the sheet, remove the twelve inch square at each corner, sew the edges together, apply the elastic, and you're done.
Since my husband won't use a top sheet either, I use duvet covers over twin comforters on our king size bed. I have learned to make these duvet covers from my unused top sheets.
I've made lots of baby crib sheets this way with special "baby" fabric, and the new moms always are so pleased to have especially, flannel sheets for winter. Flannel can usually be purchased wide enough for the 36 inch wide, by 6 inch thick, plus the four inches overhang (two each side) standard baby crib size mattress.