Tony, I suggest that you consider using Panaracer Flataway liners which are made from Kevlar and are nigh impossible to penetrate.
Unlike conventional 'rubber' or plastic liners that provide bulk between the tyre and tube, the Flataways provide a shield that few sharp objects will manage to get through.
Flataway liners are glued to the inside of the tyre, effectively becoming part of the tyre. You might need to add a smear of contact adhesive to help them stay in place!
… David