This is great. Thank you. I wish I had known this elastic years ago. I would like to add to it.
Years ago I worked at a knit fabric shop and we sold tricot fabric. It is 108” wide and makes the nicest sheets I ever had, but the heavier tricot was hard to get even then. I measured the mattress top+side+turn under the same as you, for top and bottom sheet. I cut the all corners out of the bottom sheet and the foot of the top sheet. I left the extra legend of the top sheet for a deep hem and inserted wide lace encased inside the fold back hem to give it more body and beauty. Then hem the sides of the top down to about 24” from the foot. Joined the top and bottom and about 24” up the sides of the foot. Then sew elastic all the way around the bottom sheet. The sheets are now one unit, very light weight, slick, and dry in no time. It is easy to put on and off as a unit.