I’m posting my pattern here. Easy to sew, 2D seams only. Previously I developed a real 3D pattern, but it was pain to sew and there’s practically no difference in the fit.
In the pattern, there are the two pieces for the front part with thumb (palm + thumb, wrist + thumb). Sew these together first. To make the third part, the back, just trace the palm and wrist parts together with the thumb folded away.
As it is, the size is very large – intended to fit over warm gloves. Scale it down according to your measurements or try a cheap prototype first. Going down to 85% worked well for thin rain mitts worn over a light liner (my glove size is 10).