Back in 2006 Tim Novak and I made fiberglass luges and raced them at Madison County that summer. They totally work if you're willing to do the finer details.
Practicality depends on your design. You can truly make anything you imagine. Hindsight being what it is, I can thing of dozens of improvements to make if I were inclined to make another one.
We built them in Tims basement. Be prepared for huge messes like foam dust, resin spills, etc. God bless the inventor of the shop vac.
We built them to test our creativity and have some SLICK luges. We used home insulating foam as a core with a plywood spine for added sturdiness and flat surfaces to mount trucks to.
Would I ever build another one? Yes, if it weren't for the fact that I ride a Skateluge Noreaster now and am crazy about it!!! I've ridden all kinds of luges for 17 years now and can say fiberglass does perform.