Just cut a hole in the existing storage lid at what ever angle you want the gauge to sit, cover the gauge in cling wrap and sit it in there. Then you will need some fibreglass repair kit. Wrap the fibreglass around the gauge to get the rough shape for the gauge to sit in. Then finish it off with body repair filler, sand it, prime it and paint it.

It takes ages to get a good finish on it, but the end result looks good.