If you have someone available to cut and fold 2mm cold rolled steel and you are handy with welding and the like, it is not too difficult to make your own moulds to rotomould tanks to any design you want.
This is my latest tank. It holds 130L. Moulded in polyethylene at any wall thickness you want, it can be used for water or diesel.

Various threaded fittings can be spin welded in wherever required.
These tanks are light, seamless and extremely tough. No additional stone protection is required.
Sell a couple to your mates to offset the mould costs.
Cheers,
Peter
OKA196 motorhome