hi Craig , you would be better either lining them , but in a box weld style , which is very easy to do , or fibre glass them , as this will seal and make the whole thing stronger , the only problem with glassing is you need low humidity and high temps (15c+) to work the glass and chems properly , also it will need to be washed down , as the wax you add to the mix is in styrein , so need removing .
if i was you , id go for pvc liner and get a tube of glue , cut and glue as you go , as it can come out very neat and tidy
steve