Something flat like what's linked is actually not difficult to DIY. You buy premade sheets from a place like Diamond Plate, tape them to the inside of the door, trace the outline with a marker, then cut. For the best fit in a concave door, trace a coarse outline and do a rough cut of it, then attach it, then trace the outline while it's installed and do a precise cut on that.
Something that is contoured would be significantly more difficult to make and truly not worth the effort.
That said, I'd second what wbrunner said. Just remove the speakers if you want to save weight but don't want the look of a stripped interior. The door card itself weighs almost nothing - I'd be surprised if they're even 2lbs each. If I were in your position, I'd either remove it entirely and embrace the #becauseracecar look, or I'd remove the speakers (leave the grills in place) but leave the rest.
If you want to go nuts, you could also buy a set of Euro manual crank windows & matching interior parts. Probably save 10 more lbs that way plus the windows still work when needed.
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